$50ml$ of $1M$ oxalic acid is shaken with $0.5 g$ wood charcoal. The final concentration of the solution after adsorption is $0.5M$. The amount of oxalic acid adsorbed per gram of carbon is gram.

Number of moles in $50ml$ of $1M$ oxalic acid is,

$\frac{50 \mathrm{L}}{100}\times 1 \mathrm{M}=0.05 \mathrm{moles}$

After adsorption, the moles of oxalic acid which left equal to,

$\frac{50 \mathrm{L}}{100}\times 0.5 \mathrm{M}=0.025 \mathrm{moles}$

Therefore the moles of oxalic acid adsorbed will be,

$0.05 \mathrm{moles}-0.025 \mathrm{moles}=0.025 \mathrm{moles}$

If 0.025 moles of oxalic acid adsorbs on 0.5 g of charcoal, then the moles of oxalic acid adsorbed by 1 g of charcoal:

$\mathrm{moles} \mathrm{of} \mathrm{oxalic} \mathrm{acid}=\frac{0.025 \mathrm{moles}}{0.5 \mathrm{g}}\times 1 \mathrm{g}=0.05 \mathrm{moles}$

The amount of oxalic acid will adsorbed per gram will be,

$0.05 \mathrm{moles}\times 126 \mathrm{g}/\mathrm{mol}=6.30 \mathrm{g}$