Everything Science Grade 11

CHAPTER 6. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE 6.7

Example 15: Concentration Calculations 2

QUESTION

You have a 1 dm^3 container in which toprepare a solution of potassium permanganate

(KMnO 4 ). What mass of KMnO 4 is needed to make a solution with a concentration of 0.2

M?

SOLUTION

Step 1 : Calculate the number of moles of KMnO 4 needed.

C =

n

V

therefore

n = C× V = 0. 2 × 1 = 0. 2 mol

Step 2 : Convert the number ofmoles of KMnO 4 to mass.

m = n× M = 0. 2 × 158 .04 = 31. 61 g

The mass of KMnO 4 that is needed is 31.61 g.

Example 16: Concentration Calculations 3

QUESTION

How much sodium chloride (in g) will one needto prepare 500 cm^3 of solution with a con-

centration of 0.01 M?

SOLUTION

Step 1 : Convert all quantities into the correct units forthis equation.

V =

500

1000

= 0. 5 dm^3

Step 2 : Calculate the number of moles of sodium chloride needed.

n = C× V = 0. 01 × 0 .5 = 0. 005 mol

Step 3 : Convert moles of KMnO 4 to mass.

m = n× M = 0. 005 × 58 .45 = 0. 29 g