91Ó°ÊÓ

Reaction of dolomite and HCl forms calcium, magnesium, chlorine ions; water and carbon dioxide. The balanced equation can be written as

$\mathrm{CaMg}{\left({\mathrm{CO}}_3\right)}_2\left(\mathrm{s}\right)+4\mathrm{HCl}\left(\mathrm{aq}\right)→{\mathrm{Ca}}^{2+}\left(\mathrm{aq}\right)+{\mathrm{Mg}}^{2+}\left(\mathrm{aq}\right)+4{\mathrm{Cl}}^{−}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+2{\mathrm{CO}}_2\left(\mathrm{g}\right)$

According to the question,

Volume of HCl = 33.56mL = 0.03356L

Molarity of HCl = 0.2516M

Again you know,

$\mathrm{moles}=\mathrm{molarity}×\mathrm{volume}$

Now, moles of HCl

$\begin{array}{l}=(0.2516×0.03356)\left(\mathrm{mol}\right)\\ =0.00844\left(\mathrm{mol}\right)\end{array}$

Hence, moles of HCl are 0.00844mol.

According to the balanced equation

$\mathrm{CaMg}{\left({\mathrm{CO}}_3\right)}_2\left(\mathrm{s}\right)+4\mathrm{HCl}\left(\mathrm{aq}\right)→{\mathrm{Ca}}^{2+}\left(\mathrm{aq}\right)+{\mathrm{Mg}}^{2+}\left(\mathrm{aq}\right)+4{\mathrm{Cl}}^{−}\left(\mathrm{aq}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+2{\mathrm{CO}}_2\left(\mathrm{g}\right)$

1 mole of CaMg(CO₃)₂ reacts with 4 moles HCl

Now, moles of CaMg(CO₃)₂

$\begin{array}{l}=0.00844×\frac{1}{4}\left(\mathrm{mol}\right)\\ =0.00211\left(\mathrm{mol}\right)\end{array}$

Hence, moles of CaMg(CO₃)₂ are 0.00211mol.

Moles of CaMg(CO₃)₂ = 0.00211moles

Molecular mass of CaMg(CO₃)₂= 184.305g/mol

Again you know,

$\mathrm{mass}=\mathrm{moles}×\mathrm{mass}\left(\mathrm{molar}\right)$

Mass of CaMg(CO₃)₂

_{$\begin{array}{l}=0.00211×184.305\left(\mathrm{g}\right)\\ =0.3889\left(\mathrm{g}\right)\end{array}$}

Hence, mass of CaMg(CO₃)₂ is 0.3889g.

Again you know,

Mass percent of CaMg(CO₃)₂

$\begin{array}{l}=\frac{\mathrm{mass}\left[\mathrm{CaMg}{\left({\mathrm{CO}}_3\right)}_2\right]}{\mathrm{mass}\left(\mathrm{sample}\right)}×100\mathrm{\%}\\ =\frac{0.3889}{13.86}×100\mathrm{\%}\\ =2.806\mathrm{\%}\end{array}$

Hence, mass percent of CaMg(CO₃)₂ is 2.806%.

Hence, the mass percent of dolomite in the soil is 2.806%.