Chemistry Question: How many moles of KCl are present in 75.0 mL of 2.10 M KCl?

Molarity = (No. of moles of solute) / (Volume of solution in L)

No. of moles of KCl
= (Molarity of KCl) × (Volume of solution in L)
= (2.10 mol/L) × (75.0/1000 L)
= 0.158 mol (to 3 sig. fig.)

How many moles.....

Another example of how to use the unit-factor method. Molarity is a ratio of two quantities which refer to the same "thing". Any ratio can be used as conversion factor.

0.0750 L x (2.10 mol KCl / 1L) = 0.158 mol KCl

Notice that L (liters) cancel out, leaving the units of moles. Also pay attention to precision. Both the volume and molarity are given to three significant digits, therefore, the answer is given to three significant digits.

Molarity (M) is defined as Moles/Liter. So (2.1 mol KCL/1L) * (0.075L/1)= 0.20 mols with correct sig figs. Since liters cancel with liters and mols is left on the top.