Calculate the percent composition by mass of each element in KClO 3

Question

%K:

%Cl

%O

Answer 1

The general equation for calculating the percent by mass of any element in a compound is:

% by mass element = [(mass of element in 1 mol of compound) ÷ (molar mass of compound)] x 100

1) Using the periodic table, we can find the atomic mass for each element:

K = 39.0983 g

Cl = 35.453 g

O = 15.9994 g

2) If we are assuming we have 1 mol of KClO₃, we can calculate the mass of each element in 1 mol of compound by multiplying its atomic mass by the subscript next to that element. Note: the subscript represents how many moles of that element are present in the compound.

a) The subscript for K is 1. Therefore,

mass of K = (atomic mass of K) x (subscript of K) = (39.0983 g) x (1) = 39.0983 g

b) The subscript for Cl is 1. Therefore,

mass of Cl = (atomic mass of Cl) x (subscript of Cl) = (35.453 g) x (1) = 35.453 g

c) The subscript of O is 3. Therefore,

mass of O = (atomic mass of O) x (subscript of O) = (15.9994 g) x (3) = 47.9982 g

3) Next, we calculate the mass of the compound by adding all the masses in step 2:

mass of KClO₃ = (mass of K) + (mass of Cl) + (mass of O)

= (39.0983 g) + (35.453 g) + (47.9982 g)

= 122.5495 g

4) Finally, we can calculate the percent composition by mass for each element using the first formula mentioned:

% by mass element = [(mass of element in 1 mol of compound) ÷ (molar mass of compound)] x 100

a) % mass of K = [(mass of K) ÷ (mass of KClO₃)] x 100

= [(39.0983 g K) ÷ (122.5495 g KClO₃)] x 100

= [(.3190)] x 100

= 31.90 %

b) % mass of Cl = [(mass of Cl) ÷ (mass of KClO₃)] x 100

= [(35.453 g K) ÷ (122.5495 g KClO₃)] x 100

= [(.2893)] x 100

= 28.93 %

c) % mass of O = [(mass of O) ÷ (mass of KClO₃)] x 100

= [(47.9982 g O) ÷ (122.5495 g KClO₃)] x 100

= [(.3917)] x 100

= 39.17 %

5) Last, we need to make sure the percent compositions add up to 100%:

a) (% mass of K) + (% mass of Cl) + (% mass of O) = 100%

b) (31.90%) + (28.93%) + (39.17%) = 100%

c) 100% = 100%