Here's how to calculate the number of moles of Na2CO3:

1. Find the molar mass of Na2CO3:

* Na (Sodium) has a molar mass of 22.99 g/mol

* C (Carbon) has a molar mass of 12.01 g/mol

* O (Oxygen) has a molar mass of 16.00 g/mol

So, the molar mass of Na2CO3 is: (2 * 22.99) + 12.01 + (3 * 16.00) = 105.99 g/mol

2. Use the formula:

* Moles = Mass / Molar Mass

3. Plug in the values:

* Moles = 0.75 g / 105.99 g/mol

* Moles ≈ 0.00707 mol

Therefore, 0.75 g of Na2CO3 is approximately 0.00707 moles.