Test: Stoichiometry - MCAT MCQ

The first step in solving this problem is to balance the reaction.

The balanced reaction is 

11 mol of glucose weighs 180 grams. The total weight of the products must be equal to this weight due to the conservation of mass.

Based on the balanced reaction, 2 mols of C₂H₅OH, are produced, weighing 92 grams.

The total weight of carbon dioxide is thus 180 − 92 = 88 grams.

First convert the mass of Ca(OH)₂, into mols by dividing the amount (in grams) by the molar mass.

This results in 2 mol Ca(OH)₂.

The next step in solving this problem is to balance the reaction.
The balanced reaction is

Thus for every initial mol of Ca(OH)₂ 2 mol HCl are consumed.
The reaction consumes 4 mol of HCl.

Pick an arbitrary initial mass of 100 grams. The given analysis would suggest that this contains 36 g carbon, 6 g hydrogen, and 48 g oxygen.

Convert these masses into mols by dividing them by the known molar masses of the various elements: 36/12 = 3 mol carbon, 6/1 = 6  mol hydrogen, 48/16 = 3  mol oxygen

Divide the three mol counts by their greatest common denominator to obtain the empirical formula: C₁H₂O₁

The molecular formula gives the actual chemical structure, rather than just the minimum molar ratio given by the empirical formula.
The empirical formula, based on the mol ratio, is C₁H₂O₁.This can be converted to a molar mass.

The molar mass if the molecular formula was the same as the empirical formula would be (1mol × 12g/mol) + (2mol x 1g/mol) + (1mol x 16g/mol) = 30g/mol

The actual molar mass is six times the molar mass of the empirical formula; thus the molecular formula is C₆H₁₂O₆

The first step is to determine the balanced combustion reaction.

The correct reaction is 

From Avogadro’s law, 44 liters of butane is 2 mol, which would produce 8 mol of carbon dioxide.

The molar mass of carbon dioxide is 12 + 2 × 16 = 44 g/mol, suggesting that  8 × 44 = 350 grams of carbon dioxide is released during the reaction.

The balanced reaction is 2Na + Cl₂​ → 2NaCl

Divide 12 × 10²³, atoms by Avogadro’s number to yield 2 mol of sodium entering the reaction

From the balanced reaction, 2 mol of sodium will yield 2 mol of sodium chloride

From the molar mass, 2 mol of sodium chloride corresponds to 120 grams

If all 4 mols of  Y react completly, then X is in excess because a < b. Thus only b matters for determining the reaction yield.

The ratios of coefficients in a balanced reaction gives the ratio of moles of each reagent and product required in a perfectly stoichiometric reaction.

The number of moles of product can be converted to the number of molecules by multiplying by Avogadro’s number

The correct formula is 

First determine the balanced reaction for the combination of hydrogen and oxygen to create water

The balanced reaction is 2H₂​ + O₂​ → 2H₂​O

The reaction thus requires twice as many moles of hydrogen as oxygen. Thus for equal amounts of H_2​ and O₂​, only half the oxygen will be used.

The coefficients predict that, for .5 mol of O₂ consumed, 1 mol of H₂O will result

The empirical formula is the simplest integer number of molecules that captures the molar ratio present in a sample of a compound.

This does not mean that the formula reduces until one of the subscripts is one. For example, C₃​D₄​H₂​ is a valid empirical formula.

The molecular formula can reduce to the empirical formula, but not vice versa.

For the molecular formula, each subscript is equal to or greater than its counterpart in the empirical formula.

Even in a perfectly-balanced reaction, small amounts of unreacted reagents will remain after the reaction due to heterogeneity.
The relative amount of leftover reagents changes if one of the reagents is in excess.
If there is an excess of calcium, less chlorine will remain because any unreacted chlorine is more likely to react with the extra calcium.