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WAEC Chemistry 2021 Paper

Question : 37

Total: 50

An element,

Q

69\%

^{63}Q

31\%

^{65}Q

. What is the relative atomic mass of Q ?

Solution:

Concept:

Relative atomic mass is the weighted average of all isotopic masses, using their percentage abundances as weights.

Explanation:

Multiply each isotopic mass by its percentage abundance.

Add the products and divide by

100

Relative atomic mass

= \frac{(69 \times 63) + (31 \times 65)}{100}

= \frac{4347 + 2015}{100}

= \frac{6362}{100} = 63.62

Rounded to one decimal place, this becomes

63.6

Answer:

B. 63.6

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