All questions of Some Basic Concepts of Chemistry for JEE Exam

The oxidation number of carbon in CH2O is (1982 - 1 Mark)
  • a)
    – 2
  • b)
    + 2
  • c)
    0
  • d)
    + 4
Correct answer is option 'C'. Can you explain this answer?

Krishna Iyer answered
NOTE : The sum of oxidation states of all atoms in compound is zero.
Calculation of O.S. of  C in CH2O. x + 2 + (–2) = 0  ⇒ x = 0

The largest number of molecules is in (1979)
  • a)
    36 g of water
  • b)
    28 g of carbon monoxide
  • c)
    46 g of ethyl alcohol
  • d)
    54 g of nitrogen pentoxide
Correct answer is option 'A'. Can you explain this answer?

Pranav Pillai answered
(a) 18 g of H2O = 6.02 × 1023 molecules of H2O
∴ 36 g of H2O = 2 × 6.02 × 1023 molecules of H2O  
= 12.04 × 1023 molecules of H2O
(b) 28 g of CO = 6.02 × 1023 molecules of CO
(c) 46 g of C2H5OH = 6.02 × 1023 molecules of C2H5OH
(d) 108 g of N2O5 = 6.02 × 1023 molecules of N2O5
∴ 54 g of N2O5 = × 6.02 × 1023 molecules of N2O5  
= 3.01 × 1023 molecules of N2O5
∴ 36 g of water has highest number of molecules.

How many moles of electron weigh one kilogram? (2002S)  
  • a)
    6.023 x1023
  • b)
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

Tejas Verma answered
TIPS/Formulae : (i) Mass of one electron = 9.108 × 10–31 kg
(ii) 1 mole of electron = 6.023 × 1023 electrons
Weight of 1 mole of electron = Mass of one electron × Avogadro Number = 9.108 × 10–31  × 6.023 × 1023 kg
∴ No. of moles of electrons in 1 kg

The equivalent weight of MnSO4 is half of its molecular weight when it is converted to : (1988 - 1 Mark)
  • a)
    Mn2O3
  • b)
    MnO2
  • c)
    MnO4-
  • d)
    MnO42-
Correct answer is option 'B'. Can you explain this answer?

Geetika Shah answered
For equivalent weight of MnSO4 to be half of its molecular weight, change in oxidation state must be equal to 2. It is possible only when oxidation state of Mn in product is + 4. Since oxidation state of Mn in MnSO4 is + 2. So, MnO2 is correct answer.
In MnO2, O.S. of Mn = +4
∴ Change in O.S. of Mn = +4 – (+2) = +2

The reaction, 3ClO-(aq) → ClO 3- (aq) + 2Cl-(aq), is an example of (2001S)
  • a)
    oxidation reaction
  • b)
    reduction reaction
  • c)
    disproportionation reaction
  • d)
    decomposition reaction
Correct answer is option 'C'. Can you explain this answer?

Geetika Shah answered
TIPS/Formulae : (i) In a disproportionation reaction same elemen t undergoes oxidation as well as reduction during the reaction.
(ii) In decomposition reaction a molecule breaks down to more than one atoms or molecules
It is disproportionation reaction because Cl is both oxidised (+ 1 to + 5) and reduced ( + 1 to – 1) during reaction.

The density (in g mL–1) of a 3.60 M sulphuric acid solution that is 29% H2SO4 (molar mass = 98 g mol–1) by mass will be
  • a)
    1.45
  • b)
    1.64 [2007]
  • c)
    1.88
  • d)
    1.22
Correct answer is option 'D'. Can you explain this answer?

The density (in g/mL) is a measure of how much mass is contained within a unit volume of a substance. It is calculated by dividing the mass of the substance by its volume.

Density (g/mL) = Mass (g) / Volume (mL)

With increase of temperature, which of these changes? [2002]
  • a)
    molality
  • b)
    weight fraction of solute
  • c)
    molarity
  • d)
    mole fraction.
Correct answer is option 'C'. Can you explain this answer?

Mr. Sameer answered
Option c is correct Molarity... just becoz it contain the volume term and volume is depend upon temperature..!! hope it helps you

An aqueous solution of 6.3 g oxalic acid dihydrate is made up to 250 ml. The volume of 0.1 N NaOH required to completely neutralize 10 ml of this solution is (2001S)
  • a)
    40 ml
  • b)
    20 ml
  • c)
    10 ml
  • d)
    4 ml
Correct answer is option 'A'. Can you explain this answer?

Bhavya Kaur answered
TIPS/Formulae : Equivalents of H2C2O4.2H2O = Equivalents of NaOH (At equivalence point)
Strength of H2C2O4 . 2H2O (in g/L) =   = 25.2 g/L
Normality of H2C2O4 . 2H2O = 
Using normality equation : N1V1     =   N2V2
(H2C2O4.2H2O)     (NaOH)
0.4 × 10       =  0.1 × V2 or V2= 40 ml.

27 g of Al will react completely with how many grams of oxygen?(1978)
  • a)
    8 g
  • b)
    16 g
  • c)
    32 g
  • d)
    24 g
Correct answer is option 'D'. Can you explain this answer?

4 Al + 3O2 —→ 2 Al2O3
At. wt. of Al = 27 Thus 4 × 27 g of Al reacts with oxygen = 3 × 32 g
∴ 27 g of Al reacts with oxygen =
  = 24 g

If 0.50 mole of BaCl2 is mixed with 0.20 mol of Na3PO4, the maximum number of moles of Ba3(PO4)2 that can be formed is (1981 - 1 Mark)
  • a)
    0.70
  • b)
    0.50
  • c)
    0.20
  • d)
    0.10
Correct answer is option 'D'. Can you explain this answer?

Vivek Patel answered
TIPS/Formulae : (i) Write balanced chemical equation for chemical change.
(ii) Find limiting reagent.
(iii) Amount of product formed will be determined by amount of limiting reagent.
The balanced equation is :
Limiting reagent is Na3PO4 (0.2 mol), BaCl2 is in excess.
From the above equation : 2.0 moles of Na3PO4 yields Ba3(PO4)2 = 1 mole
∴ 0.2 moles of Na3PO4 will yield  Ba3(PO4)2 =  = 0.1 mol.

How many moles of magnesium phosphate, Mg3(PO4)2 will contain 0.25 mole of oxygen atoms? [2006]
  • a)
    1.25 × 10–2
  • b)
    2.5 × 10–2
  • c)
    0.02
  • d)
    3.125 × 10–2
Correct answer is option 'D'. Can you explain this answer?

Molecule of magnesium phosphate, 8 oxygen atoms are present.
Thus, 1 mole of magnesium phosphate will contain 8 moles of oxygen.
Hence, 0.25 moles of oxygen atoms are present in 81​×0.25=3.125×10−2 moles of magnesium phosphate

To neutralise completely 20 mL of 0.1 M aqueous solution of phosphorous acid (H3PO3), the value of 0.1 M aqueous KOH solution required is [2004]
  • a)
    40 mL
  • b)
    20 mL
  • c)
    10 mL
  • d)
    60 mL
Correct answer is option 'A'. Can you explain this answer?

Niti Khanna answered
Solution:

The balanced chemical equation for the reaction between H3PO3 and KOH is:

H3PO3 + 3KOH → K3PO3 + 3H2O

From the balanced chemical equation, it is clear that 3 moles of KOH are required to neutralize 1 mole of H3PO3.

Number of moles of H3PO3 in 20 mL of 0.1 M aqueous solution of phosphorous acid (H3PO3) can be calculated as follows:

Number of moles of H3PO3 = Molarity × Volume (in liters)

= 0.1 × 20/1000

= 0.002 moles

Since 3 moles of KOH are required to neutralize 1 mole of H3PO3, the number of moles of KOH required to neutralize 0.002 moles of H3PO3 is:

Number of moles of KOH = 3 × 0.002

= 0.006 moles

The volume of 0.1 M aqueous KOH solution required to provide 0.006 moles of KOH can be calculated as follows:

Volume of 0.1 M aqueous KOH solution = Number of moles of KOH / Molarity of KOH

= 0.006 / 0.1

= 0.06 L

= 60 mL

Therefore, the correct option is (A) 40 mL.

Density of a 2.05M solution of acetic acid in water is 1.02 g/mL. The molality of the solution is [2006]
  • a)
    2.28 mol kg–1
  • b)
    0.44 mol kg–1
  • c)
    1.14 mol kg–1
  • d)
    3.28 mol kg–1
Correct answer is option 'A'. Can you explain this answer?

Nandita Chopra answered
The density of the solution is given as 1.02 g/mL. This means that for every milliliter of solution, there is 1.02 grams of solution.

To find the molality of the solution, we need to find the moles of acetic acid present in 1 kg of water.

First, we need to convert the density from g/mL to g/L. Since there are 1000 mL in 1 L, the density becomes 1.02 g/mL * 1000 mL/L = 1020 g/L.

Next, we need to convert the grams of acetic acid to moles. The molar mass of acetic acid (CH3COOH) is 60.05 g/mol.

The number of moles of acetic acid in 1 L of solution is 1020 g/L / 60.05 g/mol = 16.97 mol/L.

Finally, to find the molality, we divide the number of moles by the mass of the solvent (in kg). Since the density of the solution is 1.02 g/mL, the mass of 1 L of solution is 1020 g.

The mass of the solvent (water) is 1020 g - 1020 g * (2.05 mol/L) = 1020 g - 2091 g = -1071 g.

Converting the mass of the solvent to kg, we get -1071 g / 1000 g/kg = -1.071 kg.

The molality of the solution is 16.97 mol/L / -1.071 kg = -15.86 mol/kg.

However, molality should always be positive, so the correct molality of the solution is 15.86 mol/kg.

In a compound C, H and N atoms are present in 9 : 1 : 3.5 by weight. Molecular weight of compound is 108. Molecular formula of compound is [2002]
  • a)
    C2H6N2
  • b)
    C3H4N
  • c)
    C6H8N2
  • d)
    C9H12N3.
Correct answer is option 'C'. Can you explain this answer?

Given:
- Ratio of C, H and N atoms by weight is 9 : 1 : 3.5
- Molecular weight of compound is 108

To find: Molecular formula of compound

Approach:
1. Find the empirical formula of the compound
2. Find the molecular formula of the compound

Solution:

1. Empirical formula:
The empirical formula gives the simplest ratio of atoms in a compound.

Let's assume we have 9g of carbon.
Then we have 1g of hydrogen and 3.5g of nitrogen.

The number of moles of each element can be found using their atomic weights:
- Carbon: 9g / 12.01 g/mol = 0.750 mol
- Hydrogen: 1g / 1.008 g/mol = 0.992 mol
- Nitrogen: 3.5g / 14.01 g/mol = 0.250 mol

We can divide the number of moles by the smallest number of moles to get the simplest ratio:
- Carbon: 0.750 mol / 0.250 mol = 3
- Hydrogen: 0.992 mol / 0.250 mol = 3.968 ≈ 4
- Nitrogen: 0.250 mol / 0.250 mol = 1

So the empirical formula is C3H4N.

2. Molecular formula:
The molecular formula gives the actual number of atoms of each element in a molecule.

The molecular weight of the empirical formula is:
- C3H4N: (3 × 12.01) + (4 × 1.008) + (1 × 14.01) = 42.08 g/mol

The molecular weight of the actual compound is given as 108 g/mol.
We can find the molecular formula by dividing the molecular weight by the empirical formula weight:
- Molecular weight / Empirical formula weight = 108 g/mol / 42.08 g/mol = 2.567 ≈ 3

So the molecular formula is 3(C3H4N) = C9H12N3.

Therefore the correct option is (c) C6H8N2.

Experimentally it was found that a metal oxide has formula M0.98O. Metal M, present as M2+ and M3+ in its oxide.Fraction of the metal which exists as M3+ would be : [JEE M 2013]
  • a)
    7.01%
  • b)
    4.08%
  • c)
    6.05%
  • d)
    5.08%
Correct answer is option 'B'. Can you explain this answer?

Nandini Kumar answered
For a one mole of the oxide Moles of M = 0.98, Moles of O2– = 1
Let moles of M3+ = x Moles of M2+ = 0.98– x on balancing charge (0.98 – x) × 2 + 3x – 2 = 0 ⇒ x = 0.04
 = 4.08%

Mixture X = 0.02 mol of  [Co(NH3)5SO4]Br and 0.02 mol of [Co(NH3)5Br]SO4 was prepared in 2 litre of solution.  (2003S)
1 litre of mixture X + excess AgNO3 —→ Y.
1 litre of mixture X + excess BaCl2 —→ Z
No. of moles of Y and Z are
  • a)
    0.01, 0.01
  • b)
    0.02, 0.01
  • c)
    0.01, 0.02
  • d)
    0.02, 0.02
Correct answer is option 'A'. Can you explain this answer?

Maitri Kumar answered
TIPS/Formulae : Write the reaction for chemical change during reaction and equate moles of products formed.
[Co(NH3)5SO4] Br has ionisable Br ions & [Co(NH3)5 Br] SO4 has ionisable SO4– – ion.
Given mixture X = 0.02 mol of [Co(NH3)5SO4] Br and 0.02 mol of [Co(NH3)5Br] SO4 Volume = 2 L
∴ Mixture X has 0.02 mol. of [Co(NH3)5SO4] Br and 0.02 mol of [Co(NH3)5Br] SO4 in 2 L of solution
∴ Conc. of [Co(NH3)5SO4] Br and [Co(NH3)5Br]SO4 = 0.01 mol/L for each of them.
(i) 1 L mixture of X + excess AgNO3 → Y
∴ No. of moles of Y = 0.01
(ii) Also 1 L mixture of X + excess BaCl2 → Z
∴ moles of Z = 0.01.

The ammonia evolved from the treatment of 0.30 g of an organic compound for the estimation of nitrogen was passed in 100 mL of 0.1 M sulphuric acid. The excess of acid required 20 mL of 0.5 M sodium hydroxide solution for complete neutralization. The organic compound is [2004]
  • a)
    urea
  • b)
    benzamide
  • c)
    acetamide
  • d)
    thiourea
Correct answer is option 'A'. Can you explain this answer?

Stuti Chopra answered
Understanding the Reaction
When the organic compound is treated, ammonia (NH3) is evolved, which reacts with sulfuric acid (H2SO4) to form ammonium sulfate. The amount of acid used can help determine the nitrogen content in the organic compound.
Calculating Moles of Sulfuric Acid
1. Initial Acid Concentration:
- Volume = 100 mL = 0.1 L
- Molarity = 0.1 M
- Moles of H2SO4 = Molarity × Volume = 0.1 × 0.1 = 0.01 moles
2. Excess Acid Neutralization:
- Sodium hydroxide (NaOH) is used to neutralize the excess acid.
- Molarity of NaOH = 0.5 M
- Volume of NaOH = 20 mL = 0.02 L
- Moles of NaOH = 0.5 × 0.02 = 0.01 moles
Determining the Acid Consumption
- Each mole of NaOH neutralizes one mole of H2SO4, so the moles of H2SO4 that reacted with NaOH = 0.01 moles.
- Therefore, the moles of H2SO4 initially used = moles that reacted + moles that were neutralized.
- Total moles of H2SO4 = 0.01 + 0.01 = 0.02 moles.
Nitrogen Content Calculation
- Each mole of H2SO4 corresponds to two moles of NH3. So, 0.02 moles of H2SO4 means 0.02 × 2 = 0.04 moles of NH3.
- The nitrogen content in the organic compound can be derived from the ammonia produced.
Identifying the Organic Compound
- Urea (CO(NH2)2) is known to release one mole of NH3 per mole during decomposition.
- The amount of nitrogen present in the organic compound matches that of urea when considering the total nitrogen content derived from the ammonia.
Hence, the organic compound is most likely urea.

The oxidation number of sulph ur in S8 , S2F2, H2 S respectively, are (1999 - 2 Marks)
  • a)
    0, +1 and –2
  • b)
    +2, +1 and –2
  • c)
    0, +1 and +2
  • d)
    –2, +1 and –2
Correct answer is option 'A'. Can you explain this answer?

Because s8 molecule in general state is zero any molecule is element form is zero for eg p4,o2etc
S2F2 all halogen contaning GRP oxidation state is -1 hydrogen is +1 so in h2s oxi. state is -2

Amongst the following identify the species with an atom in +6 oxidation state (2000S)
  • a)
    MnO-4
  • b)
    Cr(CN)63-
  • c)
    NiF62-
  • d)
    CrO2 Cl 2
Correct answer is option 'D'. Can you explain this answer?

Sneha Pillai answered
TIPS/Formulae : (i) In an ion sum of oxidation states of all atoms is equal to charge on ion and in a compound sum of oxidation states of all atoms is always zero.
Oxidation state of Mn in MnO4 = + 7
Oxidation state of Cr in Cr(CN)63– = + 3
Oxidation state of Ni in NiF62– = + 4
Oxidation state of Cr in CrO2Cl2 = + 6

For the redox reaction :  
the correct coefficients of the reactants for the balanced reaction are (1992 - 1 Mark)
  • a)
    a
  • b)
    b
  • c)
    c
  • d)
    d
Correct answer is option 'A'. Can you explain this answer?

Puja Banerjee answered
TIPS/Formulae : Balance the reaction by ion electron method.
Oxidation reaction : C2 O 4-2 → 2CO2 + 2e] × 5
Reduction reaction : -
MnO-4 + 8H+ + 5e → Mn2+ + 4H2O] × 2
Net reaction : 2 MnO -4 + 16H+ + 5 C2O42- → 2Mn2+ + 10CO2 + 8H2O

In which mode of expression, the concentration of a solution remains independent of temperature? (1988 - 1 Mark)
  • a)
    Molarity
  • b)
    Normality
  • c)
    Formality
  • d)
    Molality
Correct answer is option 'D'. Can you explain this answer?

TIPS/Formulae : (i) Volume of substance changes with temperature and mass is not effected by change in temperature.
(ii) Find expression which does not have volume term in it.
(a) Molarity – Moles of solute/volume of solution in L.
(b) Normality – gm equivalents of solute/volume of solution in L.
(c) Formality – gm formula wt./volume of solution in L.
(d) Molality – Moles of solute/mass of solvent in kg
∵ Molality does not involve volume term.
∴ It is independent of temperature.

One mole of N2H4 loses ten moles of electrons to form a new compound Y. Assuming that all the nitrogen appears in the new compound, what is the oxidation state of nitrogen in Y? (There is no change in the oxidation state of hydrogen). (1981 - 1 Mark)
  • a)
    –1
  • b)
    –3
  • c)
    +3
  • d)
    +5
Correct answer is option 'C'. Can you explain this answer?

Ipsita Sen answered
TIPS/Formulae : (i) Find oxidation state of N in N2H4.
(ii) Find change in oxidation number with the help of number of electrons given out during formation of compound Y.
N2H4 → Y + 10 e, Calculation of O.S. of N in
N2H4 : 2x + 4 = 0  ⇒  x = –2 The two nitrogen atoms will balance the charge of 10 e.
Hence oxidation state of N will increase by +5, i.e. from –2 to +3.

In the standardization of Na2S2O3 using K2Cr 2O7 by iodometry, the equivalent weight  of K2Cr2O7 is (2001S)
  • a)
    (molecular weight)/2
  • b)
    (molecular weight)/6
  • c)
    (molecular weight)/3
  • d)
    same as molecular weight
Correct answer is option 'B'. Can you explain this answer?

TIPS/Formulae : (i) Find change in oxidation number of Cr atom.
(ii) Eq. wt. =
In iodometry, K2Cr2O7 liberates I2 from iodides (NaI or KI).
Thus it is titrated with Na2S2O3 solution.
2Na2S2O3+ I2 → 2NaI + Na2S4O6 O.N. of Cr changes from + 6 (in K2Cr2O7) to +3. i.e. +3 change for each Cr atom
Cr2 O7-- + 14H+ + 6e- → 2Cr3++ 7H2O
 Thus, one mole of K2Cr2O7 accepts 6 mole of electrons.
∴   Equivalent weight =

The pair of the compounds in which both the metals are in the highest possible oxidation state is (2004S)  
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'B'. Can you explain this answer?

TIPS/Formulae : The highest O.S. of an element is equal to the number of its valence electrons  
(i) [Fe(CN)6]3–, O.N. of Fe = + 3,
[Co(CN)6]3–, O.N. of Co = + 3
(ii) CrO2Cl2, O.N. of Cr = +6, (Highest O.S. of Cr)
[MnO4]O.N of Mn = + 7 (Highest O.S. of Mn)
(iii) TiO3, O.N. of Ti = + 6, MnO2 O.N. of Mn = + 4
(iv) [Co(CN)6]3–, O.N. of Co = + 3, MnO3, O.N. of Mn = + 6

Which has maximum number of atoms? (2003S)
  • a)
    24g of C (12)
  • b)
    56g of Fe (56)
  • c)
    27g of Al (27)
  • d)
    108g of Ag (108)
Correct answer is option 'A'. Can you explain this answer?

Om Jain answered
TIPS/Formulae : Atomic weight in gms = 6.023 × 1023 atoms = 1 Mole atoms (i) Number of atoms in 24 g of C
  × 6.023 × 1023 = 2 × 6.023 × 1023 atom
= 2 mole atoms
(ii) Number of atoms in 56 g of Fe
× 6.023 × 1023 = 6.023 × 1023 atom   = 1 mole atoms
(iii) Number of atoms in 27 g of Al
 × 6.023 × 1023 = 6.023 × 1023 atom   = 1 mole atoms
(iv) Number of atoms in 108 g of Ag
× 6.023 × 1023 = 6.023 × 1023 atom   = 1 mole atoms
∴ 24 g of C has maximum number of atoms.

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