Q: A barbell weighs 125 lb. Using the conversion $1 \text{ lb} = 453.59 \text{ g}$, what is its mass in grams?

$125 \times 453.59 = 56,698.75 \text{ g}$

Q: Which equation correctly represents the calculation of percent composition by mass for an element in a compound?

$\text{Percent composition} = \frac{\text{number of atoms} \times \text{atomic weight}}{\text{formula weight}} \times 100$

Q: A chemical reaction produces 8.0 g of product, but the theoretical yield is 10.0 g. What is the percent yield?

$\frac{8.0}{10.0} \times 100 = 80\%$

Q: A sample contains $3.0 \times 10^{23}$ molecules of water. How many moles of water does it contain? (Avogadro’s number $= 6.022 \times 10^{23}$)

$\frac{3.0 \times 10^{23}}{6.022 \times 10^{23}} \approx 0.50$ mol

Question 1

Which of the following statements best describes the law of conservation of mass as it applies to a chemical reaction?

Accepted Answer

The total mass of reactants equals the total mass of products in a closed system.

Question 2

A student measures the mass of a sample three times and gets values of 2.01 g, 2.03 g, and 2.02 g. The true mass is 2.50 g. Which statement best describes the measurements?

Accepted Answer

The measurements are precise but not accurate.

Question 3

Which SI unit is used to measure the amount of substance, and what is its symbol?

Accepted Answer

Mole, mol

Question 4

A barbell weighs 125 lb. Using the conversion $1 	ext{ lb} = 453.59 	ext{ g}$, what is its mass in grams?

Accepted Answer

$125 	imes 453.59 = 56,698.75 	ext{ g}$

Question 5

Which of the following is an example of a physical change?

Accepted Answer

Melting ice into water

Question 6

Which equation correctly represents the calculation of percent composition by mass for an element in a compound?

Accepted Answer

$	ext{Percent composition} = \frac{	ext{number of atoms} 	imes 	ext{atomic weight}}{	ext{formula weight}} 	imes 100$

Question 7

Which prefix and symbol represent $10^{-9}$ in the metric system?

Accepted Answer

Nano, n

Question 8

Which of the following best describes an empirical formula?

Accepted Answer

The simplest whole-number ratio of atoms in a compound

Question 9

A compound has an empirical formula weight of 30 g/mol and a molecular weight of 90 g/mol. What is the whole number multiple?

Accepted Answer

$\frac{90}{30} = 3$

Question 10

Which of the following is NOT a postulate of Dalton’s atomic theory?

Accepted Answer

Atoms can be subdivided into protons, neutrons, and electrons.

Question 11

Which equation is used to calculate kinetic energy in chemistry?

Accepted Answer

$KE = \frac{1}{2} m v^2$

Question 12

Which of the following is a polyatomic ion?

Accepted Answer

Sulfate ($SO_4^{2-}$)

Question 13

Which of the following is the correct name for $N_2O_4$?

Accepted Answer

Dinitrogen tetroxide

Question 14

A chemical reaction produces 8.0 g of product, but the theoretical yield is 10.0 g. What is the percent yield?

Accepted Answer

$\frac{8.0}{10.0} 	imes 100 = 80\%$

Question 15

Which of the following is a diatomic molecule?

Accepted Answer

Oxygen ($O_2$)

Question 16

Which equation is used to convert Celsius to Fahrenheit?

Accepted Answer

$^{\circ}F = \frac{9}{5} (^{\circ}C) + 32$

Question 17

Which of the following is the correct formula for density?

Accepted Answer

$d = \frac{m}{v}$

Question 18

Which of the following best describes a limiting reagent in a chemical reaction?

Accepted Answer

The reactant that is completely consumed and determines the maximum amount of product formed

Question 19

Which of the following is the correct SI unit for energy?

Accepted Answer

Joule (J)

Question 20

A sample contains $3.0 	imes 10^{23}$ molecules of water. How many moles of water does it contain? (Avogadro’s number $= 6.022 	imes 10^{23}$)

Accepted Answer

$\frac{3.0 	imes 10^{23}}{6.022 	imes 10^{23}} \approx 0.50$ mol

CHEM 121 Exam 1 Study Guide: Matter, Measurement, Atomic Structure, and Stoichiometry

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