Test: Stoichiometry - JAMB MCQ

A balanced reaction may not have an equal number of molecules on both the sides, because molecules may combine into one or a single molecule may breakdown into two or more. IT can disassociate or undergo double decomposition.

The chemical equation of water formation is 2H₂ + O₂ → 2H₂O. Though we have 8g of hydrogen, here oxygen is the limiting reagent. So the only 4g of hydrogen can be used to produce water i.e. 36g of water. That is 2 moles.

When two solutions are combined, the resultant molarity is M₁V₁ + M₂V₂/V₁ + V₂. Consider M₁ = 1.3, M₂ = 0.8, V₁ = 100ml and V₂ = 500ml. Now resultant molarity = 1.3(100) + 0.8(500)/100 + 500 = 0.88 mol/L.

Mole fraction of a substance is given by the formula: Mole fraction = No. of moles of that substances/No. of total moles of solution. Mole fraction of oxygen here = 3/4 + 3 + 7 = 3/14 = 0.2143.

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂ is the balanced equation because the number of atoms of different elements is the same on both sides. Rest of them do not have an equal number of atoms of different elements on both sides.

Though the other substances are excess in amount than the required, each and every reactant needs to be in a fixed ratio to attain the desired product. So, thereby, the reactant that limits the quantity of the product formed is called limiting reagent and this reactant gets consumed first completely.

4Al + 3O₂ → 2Al₂O₃ is a balanced reaction because the number of atoms of different elements on both sides is equal. The correctly balanced equations of the rest are 2H₂ + O₂ → 2H₂O; Mg(OH)₂ + 2HNO₃ → Mg(NO₃)₂ + 2H₂O and N₂ + 3H₂ → 2NH₃.

The chemical equation of formation of magnesium oxide is 2 Mg_(s) + O_2(g) ® 2MgO_(s). The formula for calculation of mass percent is (mass of solute/mass of solution) x 100. Therefore 2(24)/2(24+16) = 0.6.

Molality of a solution is given y the formula; molality = number of moles of the solute/mass of the solution. So here the number of moles is 8 and the mass of the solution is 4 kg. Molality is 8/4 = 0.5 mol/kg.

The chemical balanced equation for combustion of methane is CH_4(g) + 2O_2(g) →CO_2(g) + 2H₂O_(g). From the above equation, 1 mole of methane gives 1 mole of carbon dioxide. But 20g of methane = 1.25 moles, therefore it gives 1.25 moles of carbon dioxide = 44(1.5) = 66g.