Question

Methyl red is a pH indicator. It turns red if the pH drops to 4.4 or lower, and turns yellow in solutions of pH 6.2 or higher.(a) At what concentration of H+ does methyl red turn red?(b) At what concentration of H+ does methyl red turn yellow?(c) What color will methyl red turn if the concentration of H+ is 3.2×10−4 M

Answer 1

The formula for the pH is the following:

`pH=-\log _(10)\lbrack H^+\rbrack`

Where the concentration is in mol L⁻¹.

So, if we want the formula to calculate the concentration given the pH, we have to invert it:

`\begin{gathered} pH=-\log _(10)\lbrack H^+\rbrack \\ -\log _(10)\lbrack H^+\rbrack=pH \\ \log _(10)\lbrack H^+\rbrack=-pH \\ \lbrack H^+\rbrack=10^(-pH) \end{gathered}`

Now, let's answer the items:

(a) It turns red for pH 4.4 or lower. So:

`\begin{gathered} \lbrack H^+\rbrack=10^(-pH) \\ \lbrack H^+\rbrack=10^(-4.4) \\ \lbrack H^+\rbrack\approx3.98\cdot10^(-5)mol\, L^(-1) \end{gathered}`

The methyl red turn red for a concentration of H⁺ of approximately 3.98 10⁻⁵mol L⁻¹ or higher.

(b) It turns yellow for pH 6.2 or higher. so:

`\begin{gathered} \lbrack H^+\rbrack=10^(-pH) \\ \lbrack H^+\rbrack=10^(-6.2) \\ \lbrack H^+\rbrack\approx6.31\cdot10^(-7)mol\, L^(-1) \end{gathered}`

The methyl red turn yellow for a concentration of H⁺ of approximately 6.31 10⁻⁷mol L⁻¹ or lower.

(c) In a concentration of H⁺ of 3.2 10⁻⁴mol L⁻¹, we have:

`\begin{gathered} pH=-\log _(10)\lbrack H^+\rbrack \\ pH=-\log _(10)(3.2*10^(-4)) \\ pH\approx-(-3.5) \\ pH\approx3.5 \end{gathered}`

Which is lower then 4.4, so it will turn red.