An acid -base indicator has a `Ka of 3.0 xx 10^(-5) ` The acid form of the indicator is red and the basic form is blue. Then

Options

A. pH is 4.05 when indicator is 75 % red
B. pH is 5.00 when indicator is 75% blue
C. pH is 5.00 when indicator is 75 % red
D. pH is 4.05 when indicator is 75% blue

Answer

A, B

Explanation

` K_a =([H^(+) ][In^(-)])/( [HIn]) `
` if 75% red rArr ([HI_n])/([In^(-)]) =(75)/(25) =3 `
` rArr [H^(+) ] =3 xx 10 ^(-5) xx 3 `
` pH =5- log 9 =4.05`
` if 75% blue rArr ([In^(-)])/([HI_n]) = 3 `
` [H^(+) ] = 10 ^(-5) rArr pH =5`