AP chemistry composition stoichiometry part 1

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

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

Balance the reaction: _AlBr₃ + _K₂SO₄ → _KBr + _Al₂(SO₄)₃

• 6 AlBr₃ + 1 K₂SO₄ → 6 KBr + 1 Al₂(SO₄)₃

• 2 AlBr₃ + 3 K₂SO₄ → 6 KBr + 1 Al₂(SO₄)₃

• 1 AlBr₃ + 1 K₂SO₄ → 2 KBr + 3 Al₂(SO₄)₃

• 2 AlBr₃ + 3 K₂SO₄ → 1 KBr + 2 Al₂(SO₄)₃

What is the mass of 5.43x10²⁴ molecules of BeO?

• 171 g

• 3.29 x10⁴⁸ g

• 0.361 g

• 7.83 x 10²⁵ g

⁶³Cu is 69% of the naturally occurring isotope of Cu.  If only one other isotope is present for natural copper, what is it?

• ⁵⁹Cu
• ⁶⁵Cu
• ⁶¹Cu
• ⁶²Cu

What is the percent by mass of Cu in CuBr₂?

• 50%

• 71.6%

• 28.4%

• 44.3%