Question

1 gm of Mg is burnt with 0.56 gm O2 in a closed vessel. Which reactant is left in excess and how much

Answer 1

Answer:- 0.15 grams of Mg are present in excess.

Solution:- When Mg is burned in presence of oxygen then it form it's oxide, MgO and the balanced equation is:

$2Mg+O_2\rightarrow 2MgO$

From balanced equation, Mg and oxygen react in 2:1 mol ratio. Let's convert given grams of each to moles as:

$1g Mg(\frac{1mol}{24.3g})$ = 0.0412 mol Mg

$0.56g O_2(\frac{1mol}{32g})$ = $0.0175mol O_2$

Let's calculate the moles of oxygen required to react completely with 0.0412 moles of Mg using mol ratio as:

$0.0412 mol Mg(\frac{1mol O_2}{2mol Mg})$

= $0.0206mol O_2$

From above calculations, 0.0206 moles of oxygen are required completely but only 0.0175 moles of it are available. It means oxygen is limiting and Mg is present in excess.

let's calculate moles of Mg required to react completely with 0.0175 moles of oxygen as:

$0.0175 mol O_2(\frac{2mol Mg}{1mol O_2})$

= 0.0350 mol Mg

Excess moles of Mg = 0.0412 - 0.0350 = 0.0062 mol

Let's convert these excess moles of Mg to grams on multiply by it's atomic mass as:

$0.0062 mol Mg(\frac{24.3g}{1mol})$

= 0.15g of Mg

So, 0.15 grams of Mg are present in excess.