Total number of diprotic acids among the following is ___<br>$$\text{H}_3\text{PO}_4$$, $$\text{H}_2\text{SO}_4$$, $$\text{H}_3\text{PO}_3$$, $$\text{H}_2\text{CO}_3$$, $$\text{H}_2\text{S}_2\text{O}_7$$, $$\text{H}_3\text{BO}_3$$, $$\text{H}_3\text{PO}_2$$, $$\text{H}_2\text{CrO}_4$$, $$\text{H}_2\text{SO}_3$$

The diprotic acids, with two ionisable H atoms, are $$\text{H}_2\text{SO}_4$$, $$\text{H}_3\text{PO}_3$$ (one H is bonded to P), $$\text{H}_2\text{CO}_3$$, $$\text{H}_2\text{S}_2\text{O}_7$$, $$\text{H}_2\text{CrO}_4$$ and $$\text{H}_2\text{SO}_3$$. $$\text{H}_3\text{PO}_4$$ is triprotic, and $$\text{H}_3\text{PO}_2$$ and $$\text{H}_3\text{BO}_3$$ are monobasic. So the number is 6.