Solution :
$P^H= 3 means [H^+]=10^{-3}M$
100 ml juice contanis $10^{-3}\;mole \;H^{+}$ ions
$\qquad= 10^{-3} \times 6.022 \times 10^{23}$in 1000 ml
$\therefore$ 500 ml juice contains $H^{+}$ ions $=\large\frac{10^{-3} \times 6.022 \times 10^{23} \times 500}{1000}$
$\qquad= 3.102 \times 10^{20}$