Mass percent is given by:
[tex]\begin{gathered} mass\%=\frac{mass\text{ }solute}{mass\text{ }of\text{ }solution}\times100 \\ \end{gathered}[/tex]We need to determine the mass of the solution by using the density:
[tex]\begin{gathered} \rho=\frac{mass}{volume} \\ \rho:density \\ \\ 0.611gmL^{-1}=\frac{mass}{50.0mL} \\ mass=50.0mL\times0.611gmL^{-1} \\ mass(solution)=30.55g \\ \end{gathered}[/tex]We can plug this value into the mass percent formula to determine the mass of the solute:
[tex]\begin{gathered} mass\%=\frac{mass\text{ }solute}{mass\text{ }of\text{ }solution}\times100 \\ \\ 0.139=\frac{mass\text{ }solute}{30.55g} \\ mass\text{ }solute=0.139\times30.55g \\ mass\text{ }solute=4.25g \end{gathered}[/tex]Answer: Mass of solute is 4.25g.
