Calculate the hydrogen ion concentration in the following biological fluids whose $pH$ are given below:
$(a)$ Human muscle-fluid,$6.83$
$(b)$ Human stomach fluid,$1.2$
$(c)$ Human blood,$7.38$
$(d)$ Human saliva,$6.4$

The concentration of hydrogen ions $[H^{+}]$ is calculated using the formula: $pH = -\log [H^{+}]$,which implies $[H^{+}] = 10^{-pH}$.
$(a)$ Human muscle-fluid,$pH = 6.83$
$[H^{+}] = 10^{-6.83} = 1.48 \times 10^{-7} \ M$
$(b)$ Human stomach fluid,$pH = 1.2$
$[H^{+}] = 10^{-1.2} = 6.31 \times 10^{-2} \ M = 0.063 \ M$
$(c)$ Human blood,$pH = 7.38$
$[H^{+}] = 10^{-7.38} = 4.17 \times 10^{-8} \ M$
$(d)$ Human saliva,$pH = 6.4$
$[H^{+}] = 10^{-6.4} = 3.98 \times 10^{-7} \ M$