Question

The weight of calcium oxide formed by burning 20g of calcium in excess oxygen is:
(A) 36g
(B) 56g
(C) 28g
(D) 72g

Answer 1

Hint: Because oxygen is present in excess, Ca will be the limiting reagent and product will be formed according to the amount of Ca present. Using the reaction of Ca burning in excess oxygen and unitary method find the amount of CaO that will be produced.

Complete step by step answer:
-First of let us write down the reaction involved in burning calcium in excess of oxygen.
$Ca + {O_2} \to CaO$
Here if oxygen is present in excess, then calcium will be the limiting reagent and so the product will be formed in accordance with the amount of calcium present. This reaction is a reduction reaction of calcium.
-We know that the atomic weight of Ca is 40g/mol and molecular weight of calcium oxide (CaO) is = 40 + 16
= 56g/mol
-From the above given reaction we can see that 1 mole of Ca gives 1 mole of CaO, which also means that 40g of Ca will give 56g of CaO.
The given weight of Ca is 20g and to find out the amount of CaO produces we will use the unitary method as follows:
40g Ca produces → 56g of CaO
1g of Ca produces → $\dfrac{{56}}{{40}}$g of CaO
20g of Ca produces → $\dfrac{{56}}{{40}} \times 20$g of CaO
= 28g of CaO

Hence, the amount of calcium oxide (CaO) formed by burning 20g of calcium in excess oxygen is 28g.

The correct option will be: (C) 28g

Note: Calcium oxide is also known as quicklime or burnt lime. It is an important constituent in cement making. It reacts very vigorously with water. It sometimes reacts with water to release heat which can be enough to ignite combustible materials. If it comes in contact with moist eyes or skin or is inhaled it can cause severe irritation, coughing, sneezing, burns in the respiratory canal, etc.