20mL of 0.5 M HCl and 35 mL of 0.1 N NaOH are mixed. The resulting solution will

Options

A. Be neutral
B. Be basic
C. Turn phenolphthalein solution pink
D. Turn methyl orange red

Answer

D

Explanation

(i) 20 ml of 0.5 N HCl
`0.5 N rArr 1000 m` 0.5 mole HCl is present in 20 ml
`= (20 xx 0.5)/(1000) = 1.0 xx 10^(-2)`
(ii) 35 ml of 0.1 NaOH
`0.1 N rArr 1000 ml` of 0.1 mole NaOH is 35 ml
`= (35 xx 0.1)/(1000) = 0.35 xx 10^(-2)`
`rArr (1.0 - 0.35) 10^(-2) = 0.65 xx 10^(-2)` mole HCl
`HCl = H^(+) + Cl^(-) rArr [HCl] = [H^(+)] + [Cl^(-)]`
55 ml contains `0.65 xx 10^(-2)` mole of `H^(+)` ions
`1000 ml - (0.65 xx 10^(-2) xx 10^(3))/(55) = (6.5)/(55)`
`pH = -log [H^(+)] = -log (6.5//55)`
`= log 55 - log 6.5 = 9.2`
Due to acidic nature of solutions the colour of methylorange becomes red.