Question

If a stick of chalk carbon is 3g, what is the weight of the chalk stick?

Answer 1

Hint: Except the weight of chalk no other data like height or volume of chalk. So, the best approach towards questions like this would be to think in terms of weight. Now if we think a little deep we come to the conclusion that there is some relation between atomic mass and total weight of the chalk stick. So, now to find the atomic mass we must first know the chemical composition of the chalk. The chalk is usually made of Calcium Carbonate. Therefore, the chemical composition of chalk is CaCO$_3$ .

Complete answer:
In CaCO$_3$, there is one atom of calcium, one atom of carbon and three atoms of oxygen making the atomic ratio =
The relative atomic mass of calcium =40 that of carbon =12 and oxygen = 16
So, the atomic mass ratio becomes (40\[\times \]1: 12\[\times \]1: 16\[\times \]3) = 40:12:48.
So, formula weight of calcium carbonate is 40+12+48=100
Which means Calcium Carbonate contains 40% of Calcium, 12% of Carbon and 48% of Oxygen
It is given that weight of carbon = 3g,
Now,
We know that chalk contains 12% of Carbon,
From formula,
Weight of carbon = 12% \[\times \] weight of chalk
Therefore,
3g =\[\dfrac{12}{100}\]\[\times \] weight of chalk
On solving,
Weight of chalk = 25g
So, the correct answer is 25g.

Note:
The students should not make mistakes in the units in an answer. In case of a single choice answer if both SI and CGS units are given students should select the one unit given in question or appropriate conversion should be done.