Question

Assume you want to prepare 165 ml of a 8.5 % (m/v) solution of NaCl, starting with a 21 % (m/v) NaCl solution What volume of the 21 % NaCl solution doyou need?Your Answer:

Answer 1

66.79mL of the 21% NaCl solution are needed.

Step-by-step explanation:

1st) It is necessary to use the %m/v formula to calculate the grams of the 8.5%m/v solution of NaCl that will be in 165mL:

`\begin{gathered} \%m/v=\frac{g\text{ solute}}{mL\text{ solution}}*100 \\ 8.5\%=\frac{g\text{ solute}}{165mL}*100 \\ (8.5\%*165mL)/(100)=g\text{ solute} \\ 14.025=g\text{ solute} \end{gathered}`

Now we know that we will need 14.025g of NaCl.

2nd) Using again the %m/v formula, we can calculate the volume of the 21% NaCl solution needed:

`\begin{gathered} \%m/v=\frac{g\text{ solute}}{mL\text{ solution}}*100 \\ 21\%=\frac{14.025g}{mL\text{ solution}}*100 \\ mL\text{ solution=}(14.025g)/(21\%)*100 \\ mL\text{ solution=66.79} \end{gathered}`

So, 66.79mL of the 21% NaCl solution are needed.