The concentration of 150.0 mL of Na2CO3 is found to be 30.0 ppm. What mass of Na2CO3 is dissolved in the solution if the density of the solution is 0.750 g/mL?

First use the definition of ppm unit of concentration

ppm = [grams of solute / grams of solution] * 1,000,000

From that:

grams of solute = ppm * grams of solution / 1,000,000

Now, you need to find the grams of solution, for which you use the density of the solution and its volume:

density = mass / volume = > mass = density * volume

=> mass of solution = 150.0 ml * 0.750 g / ml = 112.5 g

And, finally, you can calculate the mass of solute

grams of solute = 30 * 112.5 g / 1,000,000 = 0.003375g

Answer: 0.003375 g