Question

unit 7 mole concept exam review

1. how many molecules are in 0.45 mole of co?
2. how many moles are in 12g of co2?
3. many people use sugar to sweeten their coffee. if the chemical formula for sugar is c₁₂h₂₂o₁₁, what would the molar mass be?
4. what is the total mass of 2.8×10²³ atoms of helium gas?
5. the total number of sodium atoms in 64g of sodium is?
6. the atomic mass unit of oxygen is 15.99 on the periodic table can also represent 15.99 ______.

a) atoms of oxygen in 1 mole
b) molecules of oxygen in 1 mole
c) particles of oxygen in 1 mole
d) grams of oxygen in 1 mole

1. which of the following statements regarding water (h₂o) is true?

a.
b.
c.
d.

1. samples of one mole of silver and one mole of copper.

a) are equal in mass
b) have the same # of atoms
c) have the same molar ma
d) have the same atomic mass

1. a diamond is made of pure carbon. the diamond on an engagement ring is 0.24 moles of carbon. how many atoms of carbon are in the diamond?
2. what is the molar mass of (nh₄)₂co₃

Explanation:

Step1: Use Avogadro's number

Number of molecules = moles $\times N_A$, where $N_A=6.02\times10^{23}$
$\text{Molecules of CO} = 0.45 \times 6.02\times10^{23} = 2.709\times10^{23}$

Step2: Calculate moles of $\text{CO}_2$

Moles = $\frac{\text{mass}}{\text{molar mass}}$, molar mass of $\text{CO}_2=12+2\times16=44\ \text{g/mol}$
$\text{Moles of CO}_2 = \frac{12}{44} \approx 0.273$

Step3: Compute molar mass of sugar

Sum atomic masses: $12\times12 + 22\times1 + 11\times16$
$\text{Molar mass} = 144 + 22 + 176 = 342\ \text{g/mol}$

Step4: Find mass of helium

First find moles: $\text{Moles} = \frac{2.8\times10^{23}}{6.02\times10^{23}} \approx 0.465\ \text{mol}$; molar mass of He=4 g/mol
$\text{Mass} = 0.465 \times 4 \approx 1.86\ \text{g}$

Step5: Count sodium atoms

Moles of Na = $\frac{64}{23} \approx 2.783\ \text{mol}$; atoms = moles $\times N_A$
$\text{Atoms of Na} = 2.783 \times 6.02\times10^{23} \approx 1.675\times10^{24}$

Step6: Match oxygen atomic mass definition

1 atomic mass unit corresponds to grams per mole for 1 mole of the element.

Step7: Evaluate water statements

1 mole of $\text{H}_2\text{O}$ has $6.02\times10^{23}$ molecules and mass = $2\times1 + 16=18.02\ \text{g}$.

Step8: Compare 1 mole Ag and Cu

1 mole of any substance has $6.02\times10^{23}$ atoms; molar masses (Ag=108, Cu=64) differ, so mass and atomic mass differ.

Step9: Count carbon atoms

Atoms = moles $\times N_A$
$\text{Atoms of C} = 0.24 \times 6.02\times10^{23} = 1.445\times10^{23}$

Step10: Calculate molar mass of $\text{(NH}_4\text{)}_2\text{CO}_3$

Sum atomic masses: $2\times(14+4\times1) + 12 + 3\times16$
$\text{Molar mass} = 2\times18 + 12 + 48 = 36 + 12 + 48 = 96\ \text{g/mol}$

Answer:

1. $2.71\times10^{23}$ molecules
2. $\approx0.27$ moles
3. $342\ \text{g/mol}$
4. $\approx1.86\ \text{g}$
5. $\approx1.68\times10^{24}$ atoms
6. d) Grams of oxygen in 1 mole
7. c. $6.02\times10^{23}$ molec of water have a mass of 18.02g
8. b) have the same # of atoms
9. $1.44\times10^{23}$ atoms
10. $96\ \text{g/mol}$