using molarity to find solute mass and volume

Question

A chemist adds 120.0mL of a ×8.74 x 10^−4 mM magnesium fluoride MgF2 solution to a reaction flask. Calculate the mass in micrograms of magnesium fluoride the chemist has added to the flask. Round your answer to 3 significant digits.

Answer 1

This is mainly a problem of units, conversions and dimensional analysis.

120 ml x 1 L /1000 ml x 8.74x10^-4 mmol/L = 1.05x10^-4 mmoles of MgF₂

Molar mass of MgF2 = 62.3 g/mol = 62.3 mg/mmol

1.05x10^-4 mmoles of MgF₂ x 62.3 mg MgF₂ / mmol = 6.54x10^-3 mg MgF₂

6.54x10^-3 mg MgF₂ x 1000 ug/mg = 6.54 ug MgF₂ (3 sig.figs.)