Question

$Mg{{(OH)}_{2}}$ in the form of milk of magnesia is used to neutralize excess stomach acid. How many moles of stomach acid can be neutralized by 1gm of $Mg{{(OH)}_{2}}$ ? (Molar mass of $Mg{{(OH)}_{2}}$ = 58.33)
(A) 0.0171
(B) 0.0343
(C) 0.686
(D) 1.25

Answer 1

Hint: Hydrochloric acid is also known as stomach acid. Two moles of hydrochloric acid will react with one mole of magnesium hydroxide in the neutralization process. Molecular weight of hydrochloric acid is 36.5$ gmmo{{l}^{-}}$ .

Complete step by step answer:
- Here, we are asked about the neutralization reaction between milk of magnesia and stomach acid. We know that milk of magnesia is a solution of magnesium hydroxide and stomach acid is hydrochloric acid. Let’s write its reaction in order to find the number of moles of stomach acid neutralized in the process.
     \[2HCl+Mg{{(OH)}_{2}}\to MgC{{l}_{2}}+2{{H}_{2}}O\]
Molecular weight of HCl = Atomic weight of H + Atomic weight of Cl = 1 + 35.5 = 36.5$ gmmo{{l}^{-}}$
Now, we know that molecular weight of HCl is 36.5$gmmo{{l}^{-}}$ and that of $Mg{{(OH)}_{2}}$ is given as 58.33 $gmmo{{l}^{-}}$ .
- We can see in the reaction that two moles of HCl react with one mole of magnesium hydroxide.
So, we can write that if 58.33 gm of $Mg{{(OH)}_{2}}$ neutralizes 36.5$\times $2= 73 gm of HCl, then 1 gm of $Mg{{(OH)}_{2}}$ will neutralize $\dfrac{1\times 73}{58.33}$ = 1.2515 gm of HCl
Now, we are asked to show the amount of HCl neutralized in moles. So we will need to convert it into moles.
     \[\text{Moles of HCl = }\dfrac{\text{Weight}}{\text{Molecular weight}}\]
     \[\text{Moles of HCl = }\dfrac{1.2515}{36.5}=0.0342 moles\]
Thus, we can conclude that 1gm of magnesium hydroxide will neutralize 0.0342 moles of stomach acid.
So, the correct answer is “Option B”.

Note: Make sure that you are putting values into the equation as two moles of HCl react with one mole of magnesium hydroxide. You need to remember the atomic weight of element chlorine which is 35.5$ gmmo{{l}^{-}}$ in order to solve this example.