What mass of lithium phosphate is needed to prepare 500. ml of a solution having a lithium ion concentration of 0.125 m?

First, we must calculate the moles of lithium ions required using:
Concentration = moles / liter
500 ml = 0.5 L
0.125 = moles / 0.5
moles = 0.0625

Lithium phosphate has the formula Li₃PO₄, meaning each mole of lithium phosphate releases three moles of lithium ions. The moles of lithium phosphate required will be:
0.0625 / 3 = 0.021 mol

The mass of one mole of lithium phosphate is 116 g/mol
Mass of lithium phosphate required = 0.021 x 116
Mass of lithium phosphate required = 2.42 grams