Q: A precisely machined sphere of pure $$^{28}\mathrm{Si}$$ was used to redefine Avogadro’s number. What is the value of Avogadro’s number as currently defined?

$$6.02214076 \times 10$$^{23}$$

Q: Which of the following statements about the mole is correct?

One mole of any substance contains $$6.02214 \times 10$$^{23}$$ particles.

Question 1

A precisely machined sphere of pure $$^{28}\mathrm{Si}$$ was used to redefine Avogadro’s number. What is the value of Avogadro’s number as currently defined?

Accepted Answer

$$6.02214076 	imes 10$$^{23}$$

Question 2

Which of the following best describes why coefficients are used to balance chemical equations instead of changing subscripts?

Accepted Answer

Changing coefficients changes the amount of substances, while changing subscripts changes the identity and properties of the substances.

Question 3

What is the balanced chemical equation for the complete combustion of pentane ($$\mathrm{C_5H_{12}}$$)?

Accepted Answer

$$\mathrm{C_5H_{12} + 8O_2 \rightarrow 5CO_2 + 6H_2O}$$

Question 4

Which of the following is a decomposition reaction?

Accepted Answer

$$2\mathrm{$$KClO_3$$}(s) \rightarrow 2\mathrm{KCl}(s) + \mathrm{O_2$}(g)$$

Question 5

Calculate the formula weight of calcium chloride ($$\mathrm{$$CaCl_2$$}). ($$Atomic weights: Ca = 40.08 amu, Cl = 35.45 amu)

Accepted Answer

$$110.98$$ amu

Question 6

A compound contains 61.31% C, 5.14% H, 10.21% N, and 23.33% O by mass. What is its empirical formula?

Accepted Answer

$$\mathrm{C_7H_7NO_2}$$

Question 7

Which of the following statements about the mole is correct?

Accepted Answer

One mole of any substance contains $$6.02214 	imes 10$$^{23}$$ particles.

Question 8

What is the percent composition by mass of hydrogen in oxalic acid ($$\mathrm{H_2C_2O_4}$$)? (Atomic weights: H = 1.008, C = 12.01, O = 16.00)

Accepted Answer

$$2.24\%$$

Question 9

A 0.1000 g sample of a compound containing only C, H, and O burns to produce 0.227 g $$\mathrm{CO_2$}$$ and 0.0930 g $$\mathrm{H_2O}. $$What is the empirical formula of the compound?

Accepted Answer

$$\mathrm{C_3H_6O}$$

Question 10

Which of the following is the correct balanced equation for the reaction of solid calcium phosphate with aqueous sulfuric acid?

Accepted Answer

$$\mathrm{Ca_3$(PO_4$)_2(s) + 3H_2SO_4(aq) \rightarrow 3CaSO_4(s) + 2H_3PO_4(aq)}$$

Question 11

If the empirical formula of a compound is $$\mathrm{C_4H_8O}$$ and its molar mass is 216 g/mol, what is its molecular formula?

Accepted Answer

$$\mathrm{C$$_{12}$$H_{24}O_3$}$$

Question 12

Which of the following is a combination reaction?

Accepted Answer

$$2\mathrm{Mg}(s) + \mathrm{O_2$}(g) \rightarrow 2\mathrm{MgO}(s)$$

Question 13

Which of the following ions is a polyatomic anion?

Accepted Answer

$$\mathrm{SO_4$^{2-}} ($$sulfate ion)

Question 14

How many molecules are in 18.0 g of water ($$\mathrm{H_2O}$$)? (Molar mass of $$\mathrm{H_2O} = 18.0 g/$$mol)

Accepted Answer

$$6.022 	imes 10$$^{23}$$

Question 15

Which of the following best describes a combustion reaction?

Accepted Answer

A rapid reaction with oxygen that produces a flame.

Question 16

What is the empirical formula of a chromium oxide that is 68.4% Cr and 31.6% O by mass?

Accepted Answer

$$\mathrm{Cr_2O_3}$$

Question 17

Which of the following is the correct formula for the nitrate ion?

Accepted Answer

$$\mathrm{NO_3$^-}$$

Question 18

A sample contains $$3.011 	imes 10$$^{23}$$ molecules of $$\mathrm{$$CO_2$$}$$. How many moles of $$\mathrm{$$CO_2$$}$$ are present?

Accepted Answer

$$0.5$$ mol

Chemical Reactions, Stoichiometry, and the Mole: Study Notes for General Chemistry

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