Chemistry exercise: mass needed to prepare a solution

One of the most easy exercises in chemistry is calculating concentrations of solutions. However, for a lot of starting students or people new in the field, it remains a hassle to carry out such exercise.

Let's say we want to prepare a 0,250 M aqueous solution of calcium chloride (CaCl2). CaCl2 is a white powder at room temperature. It dissolves readily in water.

The exercise is as follows:

What mass of CaCl2 is required to prepare a 300,0 mL of a 0,250 M solution? (Molecular mass of CaCl2 = 110,98 g/mol)

Approach this step by step. First, deduct what is expected.

1. We need to calculate the amount of mass needed. Mass is in gram (g). The solution has a concentration of 0,250 M. This means we want a concentration of 0,250 Molar or = mol/liter.

2. 0,250 mol/liter. But we do not want 1 liter, we want 300 milliliter, or 0,3 liter.
Calculate the amount of mol we need: 0,250 mol/liter to 300 milliter, I calculate it as follows:

0,250 mol divide by 1000 (1 liter = 1000 milliliter). We have 0,00025/1 milliliter.
We need 300 milliliter, so multiply by 300.

0,00025 * 300 = 0,075 mol/300 milliter.

3. Now we can calculate the mass needed of CaCl2. We use the following equation to calculate the mass:

n = m/M

with n = mol; m = mass; M = molecular mass

Fill in the equation, and we get:

0,075 mol = m/110,98 g/mol
m = 0,075 mol * 110,98 g/mol
m = 8,3235 g

4. That's it! We need to weigh 8,3235 g of CaCl2 and dissolve it in 300 mL of water to obtain a 0,250 M solution.

5. Overview:

- Calculate amount of mol needed in the solution.
- Calculate the mass needed by using the mol equation.
- Dissolve the mass in the given amount.

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