Q: A sample contains 6.46 g of helium. How many atoms of helium are present in the sample? (Molar mass of He = 4.003 g/mol, Avogadro's number $$= 6.022 \times 10$$^{23}$$ atoms/mol)

$$6.46 \div 4.003 \times 6.022 \times 10$$^{23}$$ = 9.71 \times 10$$^{23}$$ atoms

Q: How many grams of potassium are present in a sample containing $$551$$ potassium atoms? (Molar mass of K = $$39.10 g/$$mol, Avogadro's number $$= 6.022 \times 10$$^{23}$$ atoms/mol)

$$551 \div 6.022 \times 10$$^{23}$$ \times 39.10 = 3.58 \times 10$$^{-21}$$ g

Q: A chemist has $$12.4$$ moles of lead (Pb). What is the mass of lead in grams? (Molar mass of Pb = $$207.2 g/$$mol)

$$12.4 \times 207.2 = 2.57 \times 10$$^{3}$$ g$

Q: How many hydrogen atoms are present in $$10.0$$ moles of urea $$[(NH_2$)_2CO]$$? (Each molecule of urea contains $$4 H $$atoms, Avogadro's number $$= 6.022 \times 10$$^{23}$$)

$$10.0 \times 6.022 \times 10$$^{23}$$ \times 4 = 2.41 \times 10$$^{25}$$ H atoms

Q: What is the percent composition by mass of hydrogen in a compound with the molecular formula $$C_9H_8O_4 ($$Aspirin)? (Atomic masses: C = $$12.011 g/$$mol, H = $$1.00794 g/$$mol, O = $$15.9994 g/$$mol)

$$\frac{8 \times 1.00794}{180.159} \times 100 = 4.48\%$$

Q: How many moles of $$I_2$$ are required to react with $$0.429$$ mol of Al according to the equation: $$2Al + 3I_2 \rightarrow 2AlI_3$$?

$$0.429 \times \frac{3}{2} = 0.644$$ mol $$I_2$$

Q: A $$0.250 L $$solution of NaCl has a concentration of $$5.30 M. $$How many grams of NaCl are present? (Molar mass NaCl = $$58.44 g/$$mol)

$$0.250 \times 5.30 \times 58.44 = 77.49 g$$

Q: How many hydrogen atoms are present in $$72.5 g of $$ethanol ($$C_2H_6O$$)? (Molar mass = $$46.0684 g/$$mol, Avogadro's number $$= 6.022 \times 10$$^{23}$$, each molecule has 6$ H atoms)

$$\frac{72.5}{46.0684} \times 6.022 \times 10$$^{23}$$ \times 6 = 5.69 \times 10$$^{24}$$ H atoms

Q: What is the percent composition by mass of nitrogen in a $$12.04 g $$sample containing $$2.85 g N$$?

$$\frac{2.85}{12.04} \times 100 = 23.7\%$$

Question 1

A sample contains 6.46 g of helium. How many atoms of helium are present in the sample? (Molar mass of He = 4.003 g/mol, Avogadro's number $$= 6.022 	imes 10$$^{23}$$ atoms/mol)

Accepted Answer

$$6.46 \div 4.003 	imes 6.022 	imes 10$$^{23}$$ = 9.71 	imes 10$$^{23}$$ atoms

Question 2

How many grams of potassium are present in a sample containing $$551$$ potassium atoms? (Molar mass of K = $$39.10 g/$$mol, Avogadro's number $$= 6.022 	imes 10$$^{23}$$ atoms/mol)

Accepted Answer

$$551 \div 6.022 	imes 10$$^{23}$$ 	imes 39.10 = 3.58 	imes 10$$^{-21}$$ g

Question 3

A chemist has $$12.4$$ moles of lead (Pb). What is the mass of lead in grams? (Molar mass of Pb = $$207.2 g/$$mol)

Accepted Answer

$$12.4 	imes 207.2 = 2.57 	imes 10$$^{3}$$ g$

Question 4

How many hydrogen atoms are present in $$10.0$$ moles of urea $$[(NH_2$)_2CO]$$? (Each molecule of urea contains $$4 H $$atoms, Avogadro's number $$= 6.022 	imes 10$$^{23}$$)

Accepted Answer

$$10.0 	imes 6.022 	imes 10$$^{23}$$ 	imes 4 = 2.41 	imes 10$$^{25}$$ H atoms

Question 5

What is the percent composition by mass of hydrogen in a compound with the molecular formula $$C_9H_8O_4 ($$Aspirin)? (Atomic masses: C = $$12.011 g/$$mol, H = $$1.00794 g/$$mol, O = $$15.9994 g/$$mol)

Accepted Answer

$$\frac{8 	imes 1.00794}{180.159} 	imes 100 = 4.48\%$$

Question 6

A $$0.355 L $$soft drink contains $$0.133$$ mol of sucrose. What is the molarity of sucrose in the beverage?

Accepted Answer

$$\frac{0.133}{0.355} = 0.375 M$$

Question 7

How many moles of $$I_2$$ are required to react with $$0.429$$ mol of Al according to the equation: $$2Al + 3I_2 \rightarrow 2AlI_3$$?

Accepted Answer

$$0.429 	imes \frac{3}{2} = 0.644$$ mol $$I_2$$

Question 8

A $$0.250 L $$solution of NaCl has a concentration of $$5.30 M. $$How many grams of NaCl are present? (Molar mass NaCl = $$58.44 g/$$mol)

Accepted Answer

$$0.250 	imes 5.30 	imes 58.44 = 77.49 g$$

Question 9

How many hydrogen atoms are present in $$72.5 g of $$ethanol ($$C_2H_6O$$)? (Molar mass = $$46.0684 g/$$mol, Avogadro's number $$= 6.022 	imes 10$$^{23}$$, each molecule has 6$ H atoms)

Accepted Answer

$$\frac{72.5}{46.0684} 	imes 6.022 	imes 10$$^{23}$$ 	imes 6 = 5.69 	imes 10$$^{24}$$ H atoms

Question 10

What is the percent composition by mass of nitrogen in a $$12.04 g $$sample containing $$2.85 g N$$?

Accepted Answer

$$\frac{2.85}{12.04} 	imes 100 = 23.7\%$$

Question 11

A solution is made by dissolving $$45.0 g of $$glycerol ($$C_3H_8O_3$$, MM = $$92.094 g/$$mol) in $$50.0 g of $$water. The density of the solution is $$1.0959 g/mL. $$What is the molarity of glycerol in the solution?

Accepted Answer

$$\frac{45.0}{92.094} \div \frac{95.0}{1.0959 	imes 1000} = 0.4886 M$$

Question 12

Balance the following chemical equation for the combustion of ethane: $$C_2H_6 + O_2$ \rightarrow CO_2$ + H_2O$$

Accepted Answer

$$2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O$$

Question 13

A $$0.315 M $$NaOH solution contains $$6.22 g of $$NaOH. What is the volume of solution in liters? (Molar mass NaOH = $$40.0 g/$$mol)

Accepted Answer

$$\frac{6.22}{40.0} \div 0.315 = 0.4936 L$$

Question 14

How many molecules of $$CO_2$$ are produced when $$0.75$$ mol of propane ($$C_3H_8) is $$combusted according to the equation: $$C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$$?

Accepted Answer

$$0.75 	imes 3 	imes 6.022 	imes 10$$^{23}$$ = 1.4 	imes 10$$^{24}$$ molecules

Question 15

Methanol burns in air according to the equation: $$2CH_3OH + 3O_2 \rightarrow 2CO_2 + 4H_2O. If$$ $$32.0 g of $$methanol are combusted, what mass of water is produced? (Molar mass CH$$_3OH$$ = $$32.0 g/$$mol, H$$_2O$$ = $$18.0 g/$$mol)

Accepted Answer

$$\frac{32.0}{32.0} 	imes 4 	imes 18.0 = 72.0 g$$

Question 16

What is the molarity of ammonium ions in a $$0.0236 M $$solution of ammonium sulfate $$((NH_4$)_2SO_4)$$?

Accepted Answer

$$2 	imes 0.0236 = 0.0472 M$$

Question 17

A student needs to add $$3.81 g of $$glucose ($$C_6H_{12}O_6$, MM = 180.2$ g/mol) to a solution. How many mL of a 2.53$ M glucose solution should be used?

Accepted Answer

$$\frac{3.81}{180.2} \div 2.53 	imes 1000 = 8.36 mL$$

Moles, Molar Mass, and Percent Composition in General Chemistry

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