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)

$$9.70 \times 10$$^{23}$$ atoms

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}$$)

$$2.41 \times 10$$^{25}$$ H atoms

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

$$5.69 \times 10$$^{24}$$ H atoms

Q: Propane ($$C_3H_8) $$combusts according to $$C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O. If$$ $$0.75$$ mol of propane is combusted, how many molecules of $$CO_2$$ are produced? (Avogadro's number $$= 6.022 \times 10$$^{23}$$)

$$1.4 \times 10$$^{24}$$ molecules

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

$$9.70 	imes 10$$^{23}$$ atoms

Question 2

How many moles of potassium atoms are present in $$3.32 	imes 10$$^{23}$$ atoms of potassium? (Avogadro's number = 6.022$$ 	imes $$10^{23}$$ atoms/mol)

Accepted Answer

$$0.551$$ mol

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

$$2.57 	imes 10$$^{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

$$2.41 	imes 10$$^{25}$$ H atoms

Question 5

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

Accepted Answer

$$5.69 	imes 10$$^{24}$$ H atoms

Question 6

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, H = 1.00794, O = 15.9994)

Accepted Answer

$$4.48\%$$

Question 7

A solution contains 0.133 mol of sucrose in 355 mL of solution. What is the molarity of sucrose in the solution?

Accepted Answer

Question 8

How many grams of NaCl are contained in 0.250 L of a 5.30 M solution? (Molar mass of NaCl = 58.44 g/mol)

Accepted Answer

$$77.49 g$

Question 9

A student needs to prepare 8.36 mL of a 2.53 M glucose solution. How many grams of glucose ($$C_6H_{12}O_6$, MM = 180.2 g/mol) are required?

Accepted Answer

$$3.81 g$

Question 10

What is the molarity of an ethanol ($$C_2H_5OH) $$solution if 1.77 g of ethanol is dissolved in 85.0 mL of solution? (Molar mass $$= 46.067 g/$$mol)

Accepted Answer

$$0.0384 M$$

Question 11

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

$$0.644$$ mol$

Question 12

Propane ($$C_3H_8) $$combusts according to $$C_3H_8 + 5O_2 ightarrow 3CO_2 + 4H_2O. If$$ $$0.75$$ mol of propane is combusted, how many molecules of $$CO_2$$ are produced? (Avogadro's number $$= 6.022 	imes 10$$^{23}$$)

Accepted Answer

$$1.4 	imes 10$$^{24}$$ molecules

Question 13

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

Accepted Answer

$$36.0 g$

Question 14

Which of the following is the correct balanced equation for the combustion of ethanol ($$C_2H_5OH$$)?

Accepted Answer

$$C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$$

Question 15

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

$$0.4886 M$$

Question 16

Which statement about balancing chemical equations is correct?

Accepted Answer

Only the coefficients in front of formulas should be changed to balance the equation.

Question 17

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

Accepted Answer

$$23.7\%$$

Question 18

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

Accepted Answer

$$0.0472 M$$

The Mole Concept and Percent Composition in General Chemistry

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