You want to make 100 mL of a 2.5 M stock solution of calcium chloride (molecular weight 110.98 g/mol). How many grams of calcium chloride do you need to weigh out in order to make this solution? Round your answer to the nearest tenth of a gram.

27.7 g

Explanation:

100 mL = 0.100 L

To solve this problem we use the definition of molarity:

M = mol / L

From the problem we're given M = 2.5 M and volume = 0.100 L. We use the above formula and calculate the required moles of calcium chloride (CaCl₂):

2.5 M = moles / 0.100 L moles = 0.25 moles CaCl₂

Finally we use the molecular weight of calcium chloride to calculate the required mass:

0.25 moles CaCl₂ * 110.98 g/mol = 27.745 g CaCl₂

Rounding to the nearest tenth of a gram the answer is 27.7 g.