Answer :
Mass percent concentration (% m/m) is given as the mass of the solute per 100 units mass of solution.
[tex]\begin{gathered} \%mm=\frac{mass\text{ }solute}{mass\text{ }solution}\times100 \\ 12\%=\frac{10.0g}{xg}\times100 \\ 0.12=\frac{10.0\text{ }g}{x\text{ }g} \\ x=\frac{10.0\text{ }g}{0.12} \\ x=83.3g \end{gathered}[/tex]Answer: 83.3g of solution must be added to 10g of KCl in order to prepare 12.0% (m/m) KCl solution.