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  4. The concentration of hydronium ions in a cup of black coffee is 1.3 × 10-5 M. What will be the pH of the coffee?

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Q. The concentration of hydronium ions in a cup of black coffee is $1.3 \times 10^{-5} M$. What will be the $pH$ of the coffee?

Haryana PMTHaryana PMT 2010Equilibrium

Solution:

Given, $\left[H_{3} O^{+}\right]=1.3 \times 10^{-5}$
$\because p H=-\log \left[H_{3} O^{+}\right]$
$=-\log \left(1.3 \times 10^{-5}\right)$
$=-\left(\log 1.3+\log 10^{-5}\right)$
$=-0.1139+5$
$\therefore p H=4.886 \approx 4.89$