What volume of 5% LiCl and water needs to be added to the 50 mL of 0.65 M LiCl solution to make up 100 mL of 0.5 M LiCl?

Question

You need to prepare 100 mL of a 0.5 M solution of LiCl. You have 50 mL of a 0.65 M LiCl solution and 20 mL of a 5% (w/v) LiCl solution. • Molar mass of LiCl = 42.39 g/mol • % (weight / volume) means mass (in g) in volume (100 mL)

Answer 1

You need to convert 0.65 M to % or convert 5% to M, so you are working in the same units.

Let's convert 5% w/v to M.

 

5% LiCl (w/v) = 5 g LiCl/100 mls

5 g x 1 mole/42.39 g = 0.118 moles/100 mls

0.118 moles/100 mls x 1000 ml/L = 1.18 M LiCl

 

So, now you have 50 ml of 0.65 M LiCl and 20 ml 0f 1.18 M LiCl and water

 

To end up with 100 ml 0.5 M you need

0.1 L x 0.5 mol/L = 0.05 moles LiCl

You have 0.05 L x 0.65 mole/L = 0.0325 moles LiCl

So you need an additional 0.05 - 0.0325 = 0.0175 moles and these will come from the 1.198 M (5%) LiCl

 

(x L)(1.18 mol/L) = 0.0175 moles

x = 0.0148 L = 14.8 mls

 

So, you would need to add 14.8 mls of 5% (1.18 M) LiCl and 35.2 mls H2O to the 50 mls 0.65 M LiCl to end up with 100 mls of 0.5 M LiCl