How many number of molecules are there in 10 grams of nitric acid (HNO3)?

Hello!
By definition, there are about  6*10^(23)  molecules in each mole of any substance. This number is called Avogadro number and is usually denoted as N_A. If we'll find the number of moles in 10 grams of nitric acid, we'll know the number of molecules by multiplying it by N_A.
Atoms of each type have its own mass. This mass, expressed in grams per mole, may be found in many sources including the periodic table. The values for H, N and O should be memorized and are about 1, 14 and 16 grams per mole, respectively. Therefore the mass of one molecule of HNO_3  is  1 + 14 + 3*16 = 63 (g/(mol)).
Hence we have  (10 g)/(63(g/(mol))) approx0.16 mol  of nitric acid, and the number of molecules is about  6*10^(23)*0.16 approx 0.95*10^(23) = 9.5*10^(22). This is the answer.