Question

Which of the following reactions is not correct according to the law of conservation of mass.

• A. $2Mg\left(s\right)+O_2\left(g\right)+\to 2MgO\left(s\right)$
• B. $C_3{H}_8\left(g\right)+O_2\left(g\right)\to C{O}_2\left(g\right)+H_{2}O\left(g\right)$
• C. $P_4\left(s\right)+5O_2\left(g\right)\to P_4{O}_{10}\left(s\right)$
• D. $C{H}_4\left(g\right)+2O_2\left(g\right)\to C{O}_2\left(g\right)+2H_{2}O\left(g\right)$

Answer 1

The correct option is B $C_3{H}_8\left(g\right)+O_2\left(g\right)\to C{O}_2\left(g\right)+H_{2}O\left(g\right)$

Analysing the options

In option (A):

Mass of reactants is equal to mass of products. As the chemical reaction is balanced on both sides. Hence, it follows law of conservation of mass.

$2Mg\left(s\right)+O_2\left(g\right)+\to 2MgO\left(s\right)$

Total mass of reactants:
$=(2\times m_{Mg})+\left(m_{O_2}\right)$
$=(2\times 24)+\left(32\right)=80\mu$

Total mass of products:
$=(2\times m_{Mgo})$
$=2\times 40=80\mu$

In option (B):

Mass of the reactants is not equal to the products. As the chemical reaction is not balanced on both sides. Hence, this reaction does not follow law of conservation of mass

$C_3{H}_8\left(g\right)+O_2\left(g\right)\to C{O}_2\left(g\right)+H_{2}O\left(g\right)$

Total mass of reactants:
$=\left(m_{C_3{H}_8}\right)+\left(m_{O_2}\right)$
$=\left(44\right)+\left(32\right)=80\mu$

Total mass of products:
$=\left(m_{C{O}_2}\right)+\left(m_{H_{2}O}\right)$
$=\left(44\right)+\left(18\right)=62\mu$

In option (C):

Mass of reactants is equal to mass of products. As the chemical reaction is balanced on both sides. Hence, it follows law of conservation of mass.

$P_4\left(s\right)+5O_2\left(g\right)\to P_4{O}_{10}\left(s\right)$

Total mass of reactants:
$=\left(m_{P_4}\right)+(5\times m_{O_2})$
$=\left(124\right)+(5\times 32)=284\mu$

Total mass of products:
$=\left(m_{P_4{O}_{10}}\right)=284\mu$

In option (D):

$C{H}_4\left(g\right)+2O_2\left(g\right)\to C{O}_2\left(g\right)+2H_{2}O\left(g\right)$



Mass of reactants is equal to mass of products. As the chemical reaction is balanced on both sides. Hence, it follows law of conservation of mass.


Total mass of reactants:
$=\left(m_{C{H}_4}\right)+(2\times m_{O_2})$
$=\left(16\right)+(2\times 32)=80\mu$

Total mass of products:
$=\left(m_{C{O}_2}\right)+(2\times m_{H_{2}O})$
$=\left(44\right)+(2\times 18)=80\mu$

Hence, from the above analysis it is concluded that option (B) is not correct according to law of conservation of mass.