The molar mass of Na2SO4 is 142.04 g/mol. This means that 1 mole of Na2SO4 contains 2 moles of Na.

The number of moles of Na2SO4 in 25 g is:

```

25 g / 142.04 g/mol = 0.176 mol

```

The number of moles of Na in 0.176 mol of Na2SO4 is:

```

0.176 mol * 2 = 0.352 mol

```

The mass of Na in 0.352 mol is:

```

0.352 mol * 22.99 g/mol = 8.08 g

```

Therefore, 25 g of sodium sulfate contains 8.08 g of Na.