Hi, Shivani. See: The average density of 80% acid = 1,272 g/ml; Mass of 1000 ml=1272 g; Mass of pure acid= 1272 * 0.8 = 1017.6 g. Thus: 80 micro litre = 0.00008 litre; 80 microliters contains 0.081408 g of acid. So, we have 0.081408 g acid in 10 ml of water.  That's: 8.1408 g/l. MW of thioglycolic acid = 92.11. Molarrity this solution = 8.1408/92.11=0.08836 mol/l

".....The calculation doesn't need to take into account that the assay is 80% as mass = density X volume does this for you......"

You are mistaken, Andrew.

Mass of 1000 ml 80% solution is 1272 g; These 1272 g of a solution composed of 1017.6 g of the acid and 254.4 g of water. Thus, 1000 ml of the starting solution contains only 1017.6 g of acid. Accordingly, 0.08 mL - 0.081408 g acid. etc.

Thank you  Sir.