Concentrations and Dilutions

Concentrations

It’s a good idea to get familiar with different types of concentration expressions that you will use in the lab. Concentrations can be expressed in different ways. Common ways include weight per weight (w/w), weight per volume (w/v), volume per volume (v/v), molarity, and normality. You should know what these are.  

1. Weight per weight (w/w) = weight (g)/100g
2. Weight per volume (w/v) = weight (g)/100 ml
3. Volume per volume (v/v) = volume (ml)/100 ml
4. Parts per million (ppm) = number of parts of a substance in another substance (e.g. in water). It is equivalent to 1 milligram per liter (mg/l) and 1 milligram per kilogram (mg/kg)
5. Molarity (M) = moles of a solute per 1 liter of solution
6. Normality (N) = gram equivalent weight per liter of solution

In an acid-base reaction (where normality is typically used), a gram equivalent weight is the weight of a substance (acid or base) needed to neutralize H⁺ or OH^– in the acid-base reaction. A fast way of calculating normality is:

Normality = Molarity x number of equivalents

In acid and base solutions, the “number of equivalents” in the acid or base is the number of H⁺ or OH^– that will be exchanged in the neutralization reaction.

Dilutions 

We generally us the following equation in dilution calculations.

C₁V₁ = C₂V₂

Where, C₁ = initiation concentration, V₁ = initial volume, C2 = final concentration, and V2 = final volume

Example: Prepare a 500 ml solution of 0.5M HCl by diluting a 6M HCl.

C₁ = 6M, V₁ = unknown, C2 = 0.5M, V2 = 500 ml

6 x V₁ = 0.5 x 500

V₁ = (0.5 x 500)/6 = 41.67 ml

Therefore you need to add 41.67 ml of 6M HCl to 500 ml of distilled water to achieve a concentration of 0.5M. 

Practice in Lab

1. Prepare 250 ml of a 0.1 N solution of NaOH from a stock of 1 N NaOH solution
2. Prepare 250 ml solution of 2% brine (NaCl solution) from a stock of 40% brine solution
3. Prepare 250 ml of a 10% sugar solution by diluting a corn syrup of known brix.