10   mL of a solution containing Na 2 CO 3 and NaHCO 3 is titrated by HCl using phenolphthalein and then, methyl orange (added after first endpoint). The first and second endpoints were found after adding 10   mL and 15   mL of N / 10   HCl , respectively. The ratio of meq of Na 2 CO 3 and NaHCO 3 in the solution is

Step-by-step solution

Let the meq of Na 2 CO 3   and   NaHCO 3 are a   and   b , respectively. Reactions taking place are as follows. With phenolphthalein, HCl + Na 2 CO 3 → NaHCO 3 + NaCl This gives, meq of HCl = meq   of   Na 2 CO 3 10 × 1 10 = a     … i Later, with methyl orange, HCl + NaHCO 3 → H 2 CO 3 + NaCl This gives, meq of HCl = total   meq   of   NaHCO 3   15 × 1 10 = a + b     … ii On solving the two equations, we get, a =