AP Chemistry stoichiometry and thermodynamics practice practice

What is the percent by mass of oxygen in MgO?

• 20%
• 40%
• 50%
• 60%

How many molecules are in 2.5 mol of NaCl?

• 1.50 x10²⁴ molecules

• 1.50 molecules

• 4.15 molecules

• 1.51 x10²⁴ molecules

A student calculated the number of moles of methane (CH₄) present in a 100g sample of a gaseous mixture. If the student determined that there’s 0.6 moles of methane present in the sample, what is the percent composition of methane in the sample?

• .096%

• 9.6%

• .9%

• 90%

A 15.67 g sample of a hydrate of magnesium carbonate was heated to drive off the water. The mass was reduced to 7.58 g. What is the formula of the hydrate?

• MgCO₃ · 5H₂O
• MgCO₃ · 4H₂O
• MgCO₃ · 6H₂O
• MgCO₃ · 1H₂O

What is the empirical formula of a compound that is 81.82% carbon and 18.18% hydrogen?

• C₃H₈

• CH₄

• C₂H₂

• C₄H₁₀

2 KClO₃ → 2 KCl + 3 O₂
How many moles of oxygen are produced when 6.7 moles of KClO₃ decompose completely?

• 6.7 mol
• 1.0 mol
• 10.1 mol
• 4.5 mol

Cl₂ + 2 KBr → Br₂ + 2 KCl
How many grams of potassium chloride can be produced from 356 g of chlorine and 356 g of potassium bromide?

• 749 g
• 223 g
• 479 g
• 814 g

B₂H₆ + 3O₂ -->2 HBO₂ + 2 H₂O

What mass of O₂ will be needed to burn 36.1 g of B₂H₆?

• 13.8 g

• 3.86 g

• 124 g

• 198.3 g

What is the percent yield of the following reaction if 145g of P₂O₅ reacts with 40 grams of water, but you only collect 112 grams of phosphoric acid?

3 H₂O + P₂O₅ -> 2H₃PO₄

• 80%

• 85%

• 77%

• 58%

Caproic acid, which is responsible for the foul odor of dirty socks, is composed of C, H, and O atoms. Combustion of a 0.225-g sample of this compound produces 0.512 g CO₂ and 0.209 g H₂O. What is the empirical formula of caproic acid?

• CH₂O

• C₂H₂O

• C₃H₆O

• C₃H₄O₂