# A solution of sodium hydroxide has a concentration of (2.1 x 10^-1)mol/L. What colour does the solution turn when a few drops of indigo carmine are added?

Jul 2, 2017

The solution will be will be yellow.

#### Explanation:

What is the pH?

["OH"^"-"] = 2.1 × 10^"-1"color(white)(l) "mol/L"

"pOH" = -log(2.1 × 10^"-1") = 0.68

$\text{pH = 14.00 - pOH = 14.00 - 0.68 = 13.34}$

What colour is the indicator?

Indigo carmine is blue at pH 11.4 and yellow at pH 13.0.

Since pH > 13.0, the indicator will be $\textcolor{\mathmr{and} a n \ge}{\text{yellow}}$.