A gaseous mixture contains oxygen and nitrogen in the ratio of 1 : 4 by weight. Therefore the ratio of their number of molecules is

Let mass of oxygen = 1 g, Then mass of nitrogen = 4 gMol.  wt.  of  N2=28  g,  Mol. wt.  of O2=32 g28  g of N2  has =  6.02  ×  1023  molecules of nitrogen4 g of N2  has =6.02 ×  102328 × 4  molecules of nitrogen=  6.02 ×  10237   molecules of nitrogen32  g of O2  has =  6.02  ×  1023  molecules of oxygen∴  1  g of O2  has= 6.02 ×  102332 × 1=6.02 ×  102332 molecuels of  oxygenThus, ratio of molecules of oxygen : nitrogen=6.02  ×  1023/326.02  ×  1023/7=7 :  32.