Question

1. How many atoms does 2.0 moles of He represent?
2. How many sodium ions are in 3.0 moles of NaCl?
3. How many molecules are in 0.25 moles of CH4?
4. How many total atoms are in 1.0 moles of H20?

Answer 1

Question:

1. How many atoms does 2.0 moles of He represent?

2. How many sodium ions are in 3.0 moles of NaCl?

3. How many molecules are in 0.25 moles of $\mathrm{CH}_4$?

4. How many total atoms are in 1.0 moles of $\mathrm{H}_2\mathrm{O}$?

Answer:

1. How many atoms does 2.0 moles of He represent?

We use the following formula to calculate the number of atoms:

$$n \text{ (mol)} = N_{\text{(number of atoms)}} / N_A$$ $$N_{\text{(He)}} = n_{\text{(mol)}} \cdot N_A$$ $$N_{\text{(He)}} = 2.0 \text{ moles} \cdot 6.02 \cdot 10^{23} = 12.04 \cdot 10^{23} \text{ atoms}$$

2. How many sodium ions are in 3.0 moles of NaCl?

$$n \text{ (mol)} = N_{\text{(number of ions)}} / N_A$$

1 molecule of NaCl contains 1 sodium ion ($\mathrm{Na}^{+}$), that's why if we have 3.0 moles of NaCl, we have 3.0 moles of $\mathrm{Na}^{+}$.

$$N_{\text{(ions)}} = n_{\text{(mol)}} \cdot N_A$$ $$N_{\text{(ions)}} = 3.0 \text{ moles} \cdot 6.02 \cdot 10^{23} = 18.06 \cdot 10^{23} \text{ ions}$$

3. How many molecules are in 0.25 moles of $\mathrm{CH}_4$?

$$n \text{ (mol)} = N_{\text{(number of molecules)}} / N_A$$ $$N_{\text{(molecules)}} = n_{\text{(mol)}} \cdot N_A$$ $$N_{\text{(molecules)}} = 0.25 \text{ moles} \cdot 6.02 \cdot 10^{23} = 1.505 \cdot 10^{23} \text{ molecules}$$

4. How many total atoms are in 1.0 moles of $\mathrm{H}_2\mathrm{O}$?

1 molecule of $\mathrm{H}_2\mathrm{O}$ contains 3 atoms (two hydrogen & one oxygen), that's why if we have 1.0 moles of $\mathrm{H}_2\mathrm{O}$, we have 3.0 moles of atoms.

$$N_{\text{(atoms)}} = n_{\text{(mol)}} \cdot N_A$$ $$N_{\text{(atoms)}} = 3.0 \text{ moles} \cdot 6.02 \cdot 10^{23} = 18.06 \cdot 10^{23} \text{ atoms}$$

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