Calculate the mass of NO3 that will react with 3 mol of Ba (OH).

Given:
n (Ba (OH) 2) = 3 mol

To find:
m (HNO3) -?

1) Write the reaction equation:
2HNO3 + Ba (OH) 2 => Ba (NO3) 2 + 2H2O;
2) Determine the amount of the substance HNO3 (taking into account the coefficients in the reaction equation):
n (HNO3) = n (Ba (OH) 2) * 2 = 3 * 2 = 6 mol;
3) Calculate the molar mass of HNO3:
M (HNO3) = Mr (HNO3) = Ar (H) * N (H) + Ar (N) * N (N) + Ar (O) * N (O) = 1 * 1 + 14 * 1 + 16 * 3 = 63 g / mol;
4) Calculate the mass of HNO3:
m (HNO3) = n (HNO3) * M (HNO3) = 6 * 63 = 378 g.

Answer: The mass of HNO3 is 378 g.