Question

A laboratory procedure calls for making 410.0mL of a 1.2M NaNO3 solution.
What mass of NaNO3 (in g) is needed?

Answer 1

The concentration of the solution is telling you how many moles per liter of solution you have. In this case 1.2 moles of NaNO3 in one liter of solution. The problem ask you to prepare no one liter but only 410mL.
You have to calculate first how many moles will be in 410mL of this 1.2 M solution an then convert that moles to g of NaNO3.
mL of solution x (1.2 moles os NaNO3/1000mL solution) x (molecular mass of NaNO3/1mole of NaNO3)

Answer 2

Convert the volume to L. Multiply by the concentration. The result is the moles of NaNO3. Multiply by the molar mass to get grams.