Solution:
Step 1: Calculate the mass of Pb.
Note: 1 micromole = [tex]\it{1 × 10^{-6}}[/tex] mole
mass of Pb = moles of Pb × molar mass of Pb
mass of Pb = [tex]7 × 10^{-6}[/tex] mol × 207.2 g/mol
mass of Pb = 0.0014504 g
Step 2: Calculate the mass of solvent.
[tex]\text{parts per million} = \frac{\text{mass of Pb}}{\text{mass of Pb + mass of solvent}} × 10^{6}[/tex]
[tex]3.09 × 10^{5} = \frac{\text{0.0014504 g}}{\text{0.0014504 g + mass of solvent}} × 10^{6}[/tex]
[tex]0.309 = \frac{\text{0.0014504 g}}{\text{0.0014504 g + mass of solvent}}[/tex]
[tex]0.309(\text{0.0014504 g + mass of solvent}) = \text{0.0014504 g}[/tex]
[tex]\text{0.00044817 g + 0.309(mass of solvent) = 0.0014504 g}[/tex]
[tex]\text{0.309(mass of solvent) = 0.0014504 g} - \text{0.00044817 g}[/tex]
[tex]\text{0.309(mass of solvent) = 0.00100223 g}[/tex]
[tex]\text{mass of solvent} = \frac{\text{0.00100223 g}}{0.309}[/tex]
[tex]\text{mass of solvent = 0.00324 g}[/tex]
Note: 1 mg = [tex]\it{1 × 10^{-3}}[/tex] g.
[tex]\boxed{\text{mass of solvent = 3.24 mg}}[/tex]
#CarryOnLearning
