DAT General Chemistry: Stoichiometry 2

Question 1

How many moles of oxygen gas are required to completely combust 2 moles of propane (C3H8)?

Accepted Answer

10 mol

Answer

Balanced equation C3H8 + 5O2 → 3CO2 + 4H2O needs 5 mol O2 per mol propane, so 2 mol requires 10 mol.

Question 2

What is the percent composition by mass of carbon in CO2 (C=12, O=16)?

Accepted Answer

27.3%

Answer

Carbon's 12 g out of CO2's molar mass of 44 g gives 12/44 = 27.3%.

Question 3

If 3 mol of N2 react with 6 mol of H2, which is the limiting reagent in N2 + 3H2 → 2NH3?

Accepted Answer

H2

Answer

3 mol N2 needs 9 mol H2 but only 6 mol is present, so H2 limits the reaction.

Question 4

How many grams are in 0.50 mol of NaCl (Na=23, Cl=35.5)?

Accepted Answer

29.25 g

Answer

NaCl molar mass is 58.5 g/mol, so 0.50 mol equals 29.25 g.

Question 5

What is the empirical formula of a compound that is 40% C, 6.7% H, and 53.3% O by mass?

Accepted Answer

CH2O

Answer

Dividing by atomic masses gives a 1:2:1 mole ratio of C:H:O, the formula CH2O.

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