Numerical: How will you prepare a 0.75% #Na_2CO_3# solution?
Here's a way you could do this.
The first thing you need to do is pick a volume for the target solution.
To make the calculations easier, I'll show you how to prepare a 0.75% w/v solution that has a total volume of 100 mL.
You can isolate the mass of the solute to get
Since you want your target solution to have a volume of 100 mL and a mass by volume percent concentration of 0.75%, the mass of solute that would be needed is
So, if you dissolve 0.75 g of sodium carbonate in enough water to make the total volume of the solution equal to 100 mL, you'll get a 0.75% w/v solution.
You can use this approach for any volume you may need the target solution to have.