Which element from following has largest atomic size as compared to other three?

Calculate the enthalpy change when 12 g carbon react with sufficient hydrogen to form methane. If enthalpy of formation of methane is $-75 \mathrm{~kJ} \mathrm{~mol}^{-1}$.

What type of ligand is the oxalate ion?

What is pH of a centimolar solution of $\mathrm{H}_2 \mathrm{SO}_4$ ?