Q: A sample contains $2.5$ moles of copper atoms. How many atoms of copper are present in the sample?

$2.5 \times 6.022 \times 10^{23} = 1.51 \times 10^{24}$ atoms

Q: How many molecules of carbon dioxide ($CO_2$) are present in a $15.0$ g sample?

$2.05 \times 10^{24}$ molecules

Question 1

Which of the following statements best describes Avogadro's number?

Accepted Answer

It is the number of particles in 1 mole of a substance, equal to $6.022 	imes 10^{23}$.

Question 2

A sample contains $2.5$ moles of copper atoms. How many atoms of copper are present in the sample?

Accepted Answer

$2.5 	imes 6.022 	imes 10^{23} = 1.51 	imes 10^{24}$ atoms

Question 3

What is the molar mass of diphosphorus pentoxide ($P_2O_5$)?

Accepted Answer

$2(30.97) + 5(16.00) = 141.94$ g/mol

Question 4

How many molecules of carbon dioxide ($CO_2$) are present in a $15.0$ g sample?

Accepted Answer

$2.05 	imes 10^{24}$ molecules

Question 5

A $38.0$ g sample of $Cu_2S$ contains $79.857\%$ copper by mass. How many grams of copper are in the sample?

Accepted Answer

$30.3$ g

Question 6

What is the empirical formula for a compound containing $25.9\%$ nitrogen and $74.1\%$ oxygen?

Accepted Answer

$N_2O_5$

Question 7

A compound has an empirical formula of $CH_3$ and a molecular mass of $42.1$ g/mol. What is its molecular formula?

Accepted Answer

$C_3H_6$

Question 8

Which of the following best describes a strong electrolyte?

Accepted Answer

A substance that conducts current very efficiently and is completely ionized in water.

Question 9

According to Henry's Law, what happens to the solubility of a gas in a liquid as the pressure of the gas above the liquid increases?

Accepted Answer

The solubility increases.

Question 10

Which statement about the effect of temperature on the solubility of gases in water is correct?

Accepted Answer

Solubility of gases in water decreases with increasing temperature.

Question 11

A solution is prepared by dissolving $500.0$ g of potassium phosphate ($K_3PO_4$) in enough water to make $1.50$ L of solution. What is the molarity of the solution? (Molar mass of $K_3PO_4 = 212.27$ g/mol)

Accepted Answer

$1.57$ M

Question 12

You have a $10.0$ M sugar solution. What volume of this solution is needed to obtain $2.00$ mol of sugar?

Accepted Answer

$0.200$ L

Question 13

A $0.50$ M solution of sodium chloride is left in an open beaker for several days. Which of the following will decrease the concentration of the salt solution?

Accepted Answer

Adding water to the solution.

Question 14

What is the new molarity when $50.0$ mL of water is added to $80.0$ mL of a $2.5$ M HCl solution?

Accepted Answer

$0.87$ M

Question 15

Which of the following is the correct formula for calculating pH?

Accepted Answer

$pH = -\log [H_3O^+]$

Question 16

A solution has $[H^+] = 3.4 	imes 10^{-4}$ M. What is its pH?

Accepted Answer

$3.47$

Question 17

Which of the following best describes Beer's Law?

Accepted Answer

It relates the absorbance of a solution to its concentration, path length, and molar absorptivity: $A = \epsilon b c$

Question 18

A blue solution absorbs light most strongly in which region of the visible spectrum?

Accepted Answer

Green

Question 19

Which of the following is true about the preparation of a standard solution for spectrophotometry?

Accepted Answer

The solution is diluted to create a range of concentrations for calibration.

Analyzing Compounds: Molar Mass, Percent Composition, and Chemical Formulas

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