Calculating Numbers of Molecules and Atoms from Mass (a) How many glucose molecules are in 5.23 g of C6H12O6? (b) How many oxygen atoms are in this sample? (a) How many nitric acid molecules are in 4.20 g of HNO3? (b) How many O atoms are in this sample?

Solution 2PE To convert from molecules to grams, it is necessary to first convert the number of molecules of a substance by dividing by Avogadro's number to find the number of moles, and then multiply the number of moles by the molar mass of this substance. Avogadro's number is given as 6.022 x 10^23. Step 1 Glucose its molecular formula is C6H12O6. The formula weight of glucose C6H12O6.=180.1559 g/mol. We know that From avogadro's law 1 mol of C6H12O6=6.023*10 .molecules of C6H12O6. So ,molecules C6H12O6=5.23g of C6H12O6.*(1 mol C6H12O6)/180.1559)*(6.023*10 molecules C6H12O6/1 mol C6H12O6). 22 =1.74 *10 molecules C6H12O6. 22 So there are 1.74*10 molecules C6H12O6 present in 5.23 g of C6H12O6. Step 2 There are six oxygen atoms in each C6H12O6 molecule . So ,atoms O=(1.74*10 molecules C6H12O6)*(3 atoms O/1 molecule C6H12O6) 22 =5.22*10 atoms ‘O’ 23 So, there are 1.20*10 atoms of oxygen present in this sample .