Q: Which of the following is the correct formula for calculating the percentage composition by mass of an element in a compound?

$$\frac{\text{mass of element}}{\text{molar mass of compound}} \times 100$$

Question 1

Which of the following best describes a chemical change?

Accepted Answer

A process that produces a new substance with different properties.

Question 2

Which equation correctly represents the combustion of methane ($$CH_4) in $$oxygen?

Accepted Answer

$$CH_4(g) + 2O_2(g$$) \rightarrow $$CO_2(g) + 2H_2O(g$$)$$

Question 3

A student mixes aqueous solutions of barium chloride and sodium sulfate. What type of change occurs and what is the evidence?

Accepted Answer

Chemical change; a white precipitate forms.

Question 4

Which of the following is the correct balanced equation for the reaction of aluminum with hydrochloric acid?

Accepted Answer

$$2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)$$

Question 5

What is the empirical formula of a compound containing 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass?

Accepted Answer

$$C_2H_4O$$

Question 6

Which of the following statements about limiting reagents is correct?

Accepted Answer

The limiting reagent determines the maximum amount of product formed.

Question 7

Calculate the number of moles in 12 g of carbon ($$^{12}C).$$

Accepted Answer

$$1$$ mol

Question 8

A solution is prepared by dissolving 5.85 g of NaCl in enough water to make 1.00 L of solution. What is the concentration of NaCl in mol/L?

Accepted Answer

$$0.100$$ mol/L

Question 9

Which of the following is NOT a sign that a chemical reaction has occurred?

Accepted Answer

Evaporation of water

Question 10

What is the percentage yield if 2.0 g of product is obtained from a reaction with a theoretical yield of 2.5 g?

Accepted Answer

$$80\%$$

Question 11

Which of the following best describes the greenhouse effect?

Accepted Answer

The trapping of heat in the Earth's atmosphere by certain gases.

Question 12

Which chemical equation correctly represents the reaction of calcium carbonate with hydrochloric acid?

Accepted Answer

$$CaCO_3(s) + 2$$HCl(aq) \rightarrow $$CaCl_2(aq$$) + $$CO_2(g) + H_2O(l$$)$$

Question 13

A sample contains 4.0 g of hydrogen and 32.0 g of oxygen. What is the empirical formula?

Accepted Answer

$$H_2O_2$$

Question 14

Which of the following is the correct formula for calculating the percentage composition by mass of an element in a compound?

Accepted Answer

$$\frac{	ext{mass of element}}{	ext{molar mass of compound}} 	imes 100$$

Question 15

Which of the following is a physical change?

Accepted Answer

Dissolving sugar in water

Question 16

If 10.0 g of propane ($$C_3H_8) is $$combusted in excess oxygen, how many grams of $$CO_2$$ are produced? ($$M_r of$$ $$C_3H_8 = 44$$, $$M_r of$$ $$CO_2 = 44$)

Accepted Answer

$$30.0 g$

Question 17

Which of the following best explains why balancing chemical equations is necessary?

Accepted Answer

To obey the law of conservation of mass.

Question 18

A solution contains 0.5 mol of $$NaCl in 2.0 L of $$water. What is the molarity of the solution?

Accepted Answer

$$0.25$$ mol/L

Question 19

Which of the following is the correct formula for calculating the number of moles from mass and molar mass?

Accepted Answer

$$n = \frac{m}{M}$$

Chemical Reactions, Equations, and Stoichiometry: Core Concepts and Applications

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