A 0.21 g of Sulfuris acid is dissolved completely in sufficient water to make 0.25 litre of final solution. Calculate the hydrogen ion concentration (in mol 1 -1) in this solution. Give your answer in scientific notation to an appropriate number of significant figures. Remember to show successive steps and explain reasoning.

Please help, I'm really stuck!!

I have answered this three times. I will be happy to critique your thinking.