Chemistry question..?

Question

You need to prepare 100ml of a 0.25 M solution of KCI.

You have 50ml of a 0.4 M KCI solution and 100mL of a 2.5% (w/v) KCI solution.

What volume of 2.5% KCI and water needs to be added to the 50mL of 0.4 M KCI solution to make up 100mL of 0.25 M KCI?

Molecular weight of KCI = 74.5

..?

Thank you :)

Answer 1

Equations you need for this are.

Molarity = moles / litres (M = mol / L)

moles = mass / molecular weight

Firstly, work out how many moles of KCl, you need to make up 100 ml of 0.25 M KCl.

Molarity = mole / Litre

therfore moles required = Molarity x Litre

= 0.25 (M) x 0.1 (L)

= 0.025 moles

So 0.025 moles of KCl are required to make up your solution of KCl. So now you add the solutions together in such a way as to get a total of 0.025 moles of KCl in the final solution.

Personally, I would start by using all the 0.4 M KCl solution. So work out how many moles of KCl are present in this solution.

moles KCl in 50ml of 0.4 M solution = Molarity x L

= 0.4 (M) x 0.05 (L)

= 0.02 moles.

So if you use all the 0.4 M siolution you have 0.02 moles. Since you need a total of 0.025 moles of KCl this means that you will have to get the other 0.005 moles of KCl by adding 2.5% KCl solution.

To work out how much you need first work out the molarity of 2.5 % w/v KCl.

% w/v means % weight per volume.

For a 2.5% solution this means that 2.5 g of KCl is dissolved in 100 ml of solution.

Now, the moles of KCl in this solution

moles = mass / molecular weight

= 2.5 / 74.5

= 0.034 moles

and the Molarity is

M = mole / L

= 0.034 (mol) / 0.1 (L)

= 0.34 M

You need an extra 0.005 moles of KCl, so work out what volume of the 2.5% (0.34M) KCl will provide 0.005 moles

M = moles/L

therefore L =  moles / M

therfore L = 0.005 (mol) / 0.34 (M)

= 0.0147 L

= 14.7 ml

Therefore 14.7 ml of 2.5% KCl solution contains 0.005 moles of KCl.

So, Combine the 50 ml of 0.4 M KCl with 14.7 ml of the 2.5 % w/v KCl and make it up to 100 ml with water to give you the required solution.