Question

1. how many molecules are in 0.25 moles of co₂?

a. $6.02 \times 10^{23}$
b. $1.51 \times 10^{23}$
c. $2.41 \times 10^{23}$
d. $3.01 \times 10^{23}$

1. how many moles are in 98.0 g of sulfuric acid (h₂so₄)?

a. 0.50 mol
b. 1.00 mol
c. 2.00 mol
d. 4.00 mol

1. how many liters are in 5.00 moles of helium gas at stp?

a. 11.2 l
b. 56.0 l
c. 22.4 l
d. 112 l

1. which substance has the greatest molar mass?

a. o₂
b. co₂
c. nacl
d. caco₃

Explanation:

Problem 20

Step1: Use Avogadro's number formula

Number of molecules = Moles × Avogadro's number
Avogadro's number = $6.02 \times 10^{23}$ molecules/mol
Expression: $\text{Molecules} = 0.25\ \text{mol} \times 6.02 \times 10^{23}\ \text{molecules/mol}$

Step2: Calculate the result

Expression: $0.25 \times 6.02 \times 10^{23} = 1.51 \times 10^{23}$

Problem 21

Step1: Calculate molar mass of $\text{H}_2\text{SO}_4$

Molar mass = $2(1.008) + 32.07 + 4(16.00)$
Expression: $\text{Molar mass} = 2.016 + 32.07 + 64.00 = 98.086\ \text{g/mol} \approx 98.1\ \text{g/mol}$

Step2: Calculate moles from mass

Moles = $\frac{\text{Mass}}{\text{Molar mass}}$
Expression: $\text{Moles} = \frac{98.0\ \text{g}}{98.1\ \text{g/mol}} \approx 1.00\ \text{mol}$

Problem 22

Step1: Use STP molar volume rule

At STP, 1 mol gas = 22.4 L
Expression: $\text{Volume} = 5.00\ \text{mol} \times 22.4\ \text{L/mol}$

Step2: Compute the volume

Expression: $5.00 \times 22.4 = 112\ \text{L}$

Problem 23

Step1: Calculate molar mass of each substance

• $\text{O}_2$: $2(16.00) = 32.00\ \text{g/mol}$
• $\text{CO}_2$: $12.01 + 2(16.00) = 44.01\ \text{g/mol}$
• $\text{NaCl}$: $22.99 + 35.45 = 58.44\ \text{g/mol}$
• $\text{CaCO}_3$: $40.08 + 12.01 + 3(16.00) = 100.09\ \text{g/mol}$

Step2: Compare molar masses

$\text{CaCO}_3$ has the highest value.

Answer:

1. B. $1.51 \times 10^{23}$
2. B. 1.00 mol
3. D. 112 L
4. D. $\text{CaCO}_3$