A 25.0 mL sample of HNO3 solution requires 35.7 mL of 0.108 M Na2CO3 to completely react with all of the HNO3 in the solution. What is the concentration of the HNO3 solution?

First, let write down the reaction equation

2HNO3 + Na2CO3 - - - > H2CO3 + 2NaNO3

find the moles of Na2CO3 in the solutions

moles = M x L---> moles Na2CO3 = 0.108 M x 0.0357 L = 0.00386 mole

then do mole-mole ratio

0.00386 mol Na2CO3 (2 mol HNO3 / 1 mol NaCO3) = 0.00771 mol HNO3

to find concentration (molarity = mol / Liters)

M = 0.00771 moles / 0.025 L = 0.308 M HNO3