Question 63908

May 20, 2015

Your solution contains 0.32 g of sodium chloride.

A percent concentration by mass expresses the ratio between the mass of the solute, in your case sodium chloride, and the total mass of the solution, multiplied by 100.

$\text{%mass" = "mass of solute"/"mass of solution} \cdot 100$

So, you know that your sample weighs 16 g and that it's 2% m/m sodium chloride. You can use this data to determine how many grams of sodium chloride must be present in that much solution by

"%mass" = m_(NaCl)/("16.0 g") * 100 = 2.0%#

${m}_{N a C l} = \frac{16.0 \cdot 2.0}{100} = \textcolor{g r e e n}{\text{0.32 g}}$

Think of it like this: you know the mass of sodium chloride represents 2% by mass of a solution. To get from 2% to 100% you need to multiply by 50, so the mass of sodium chloride can be determined by dividing the total mass by 50.

For example, if you have a 25% m/m solution that has a total mass of 16 g, the mass of sodium chloride would be 4 g.

That happens because you need to divide 100% by 4 to get to 25%, so divide the total mass by 4 to get the mass of the solute.