Solution:
1. Calculate the molar mass of SO4:
- Sulfur (S) has a molar mass of approximately 32.07 g/mol.
- Oxygen (O) has a molar mass of approximately 16.00 g/mol.
- Since there are four oxygen atoms in SO4, multiply the molar mass of oxygen by 4: 16.00 g/mol * 4 = 64.00 g/mol.
- Now, add the molar mass of sulfur and the molar mass of oxygen: 32.07 g/mol + 64.00 g/mol = 96.07 g/mol.

2. Calculate the molar mass of Na2SO4:
- Sodium (Na) has a molar mass of approximately 22.99 g/mol.
- Since there are two sodium atoms in Na2SO4, multiply the molar mass of sodium by 2: 22.99 g/mol * 2 = 45.98 g/mol.
- Add this to the molar mass of SO4 calculated earlier: 45.98 g/mol + 96.07 g/mol = 142.05 g/mol.

3. Calculate the percent composition of SO4 in Na2SO4:
- Divide the molar mass of SO4 (96.07 g/mol) by the molar mass of Na2SO4 (142.05 g/mol) and multiply by 100 to get the percentage: (96.07 g/mol / 142.05 g/mol) * 100 67.64%.

So, the percent composition of SO4 in Na2SO4 is approximately 67.64%.

Number of sodium ions = Number of chloride ions = 0.949 moles x 6.022 x 10²³ ions/mole
Number of sodium ions = 0.949 moles x 6.022 x 10²³ ions/mole 5.70 x 10²³ sodium ions
There are approximately 5.70 x 10²³ sodium ions in 0.949 moles of sodium chloride (NaCl).

The molar mass of sulfur (S) is approximately 32.06 grams per mole (g/mol). This means that 1.00 mole of sulfur has a mass of approximately 32.06 grams.

Solution:
Now, you can calculate the mass of 1.35 moles of iron (Fe):

Mass (grams) = Number of moles × Molar mass
Mass (grams) = 1.35 mol × 55.845 g/mol
Mass (grams) 75.35675 grams

So, the mass of 1.35 moles of iron (Fe) is approximately 75.36 grams.