Answer:
66.79mL of the 21% NaCl solution are needed.
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:
[tex]\begin{gathered} \%m/v=\frac{g\text{ solute}}{mL\text{ solution}}*100 \\ 8.5\%=\frac{g\text{ solute}}{165mL}*100 \\ \frac{8.5\%*165mL}{100}=g\text{ solute} \\ 14.025=g\text{ solute} \end{gathered}[/tex]
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:
[tex]\begin{gathered} \%m/v=\frac{g\text{ solute}}{mL\text{ solution}}*100 \\ 21\%=\frac{14.025g}{mL\text{ solution}}*100 \\ mL\text{ solution=}\frac{14.025g}{21\%}*100 \\ mL\text{ solution=66.79} \end{gathered}[/tex]
So, 66.79mL of the 21% NaCl solution are needed.
