Question

Important equations for making up solutions Weight of solute (g) = formula weight of solute (g/mole) x molarity (mol/l) x final volume (L) C1V1 = C2V2

How to make solutions

Choose the correct formula from above. Please show all the steps and be sure to clearly show what values are being using relating to the formula.

1. Make 80 ml of a 0.01 M KMnO₄ solution.
2. Using the 0.01 M KMnO₄ stock solution make 25 ml of a 2mM KMnO₄ solution.

Answer 1

For making 80mL of a 0.01M KMnO₄ ​​​​​​, we need to calculate the mass of KMnO₄.

Formula weight of KMnO₄= sum of atomic masses of k, Mn and O

(Atomic mass of K=39g, Mn=54.9, O =16g

Formula weight of solute (KMnO₄)= 39+54.9+16*4=39g+54.9+64=157.9g

one mole =formula weight =157.9g

So,we can write formula weight of KMnO₄ =157.9g/mol

Using first formula,calculate weight of KMnO₄

Weight of KMnO₄ (g) ^₌ formula weight of KMnO₄(g/mol)*molarity (mol/L) *Final volume(L)

Molarity of KMnO₄  =0.01M

Volume of KMnO₄= 80mL

1mL =0.001L

80mL =0.001L*80mL/1mL=0.08L

Putting all these values in the above given formula

Weight of KMnO₄= 157.9g/mol*0.01mol/L*0.08L=0.12632g

After round off, weight of solute is 0.126g

Molarity is 0.01M. Molarity is mole of solute dissolved per liter of the solution. So, instead of M we can use unit mol/liter. The units mol and L will get cancelled.

2) In second case, we are diluting the solution of KMnO₄ ​​​​​​.

Use the formula C₁V₁=C₂V₂

Here,C₁₌ initial concentration

C₂= final concentration

V_1⁼initial volume

V₂= final volume

Given C₁=0.01M

C_{2 =}2mM =2*10^-3 M (since 1mM =10^-3 M)

V₁=?

V₂= 25mL

C₁V₁=C₂V₂

V₁=C₂V₂/C₁= 2*10^-3M *25mL/0.01M=5mL

Second formula gives us the volume of KMnO4 required  to make 25mL of 2mM KMnO₄ solution.