Question

The $pH$ of blood is $7 \cdot 4$ . What is ratio the of $\dfrac{{\left[ {HP{O_4}^{2 - }} \right]}}{{{H_2}P{O_4}^ - }}$ in the blood?
Given: $K{a_1},K{a_2},K{a_3}$for ${H_3}P{O_4}$ is ${10^{ - 3}},8 \times {10^{ - 8}},{10^{ - 12}}$ respectively.
A.$2:1$
B.$1:2$
C.$3:1$
D.$1:3$

Answer 1

Hint: $HP{O_4}^{2 - }$ is called dipotassium phosphate. It is an inorganic compound with the formula ${K_2}HP{O_4}$. ${H_2}P{O_4}^ - $ is called dihydrogen phosphate. It is a monovalent inorganic anion. The formula used is $pH = p{K_a} + \log \left[ {\dfrac{{\left( {salt} \right)}}{{\left( {acid} \right)}}} \right]$

Complete step-by-step answer:
As we know the formula above, let us write it once again and define the values separately below,
$pH = p{k_a} + \log \left[ {\dfrac{{\left( {salt} \right)}}{{\left( {acid} \right)}}} \right]$
Here the value of,
$
  pH = 7 \cdot 4 \\
    \\
$
And the value of,
$p{K_a} = 7 \cdot 1$
And as we know the basic concept of salts and acid, it becomes easy to define that in the question and write it down in the formula for the further calculation,
Salt is $HP{O_4}^{2 - }$
And acid is ${H_2}P{O_4}^ - $
Now when we substitute the above known values into the formula we will get the ratio,
$ = 7 \cdot 4 = 7 \cdot 1 + \log \left[ {\dfrac{{\left( {HP{O_4}^{2 - }} \right)}}{{\left( {{H_2}P{O_4}^ - } \right)}}} \right]$
$ = \log \left[ {\dfrac{{\left( {HP{O_4}^{2 - }} \right)}}{{({H_2}P{O_4}^ - )}}} \right] = 0 \cdot 3 = \log 2$
Now hence, it becomes clear that the ratio of the concentration of $HP{O_4}^{2 - }$ and ${H_2}P{O_4}$ is $2:1$

Additional information:
the scale of pH is marked by 14 different parts starting from 1-14 it defines the concentration of hydronium ions in the compound as we know greater the hydronium ion concentration in the compound greater will be its acidic strength, therefore, 1 shows highest acidic nature as we move from 1 to 6 the acidic nature tend to decrease. 7 shows a neutral compound and from 8-14 the basic character increase hence 14 shows the highest basic strength.

Hence the right option is (A).

Note: To see the potential of hydroxide ion the scale can be reversed and subtracting the exponent of 10 with 14 we can get the concentration of hydroxide ion which is present in the solution or in the given compound.