The amount of sodium hydrogen carbonate, ${NaHCO}_{3}$ , in an antacid tablet is to be determined by dissolving the tablet in water and titrating the resulting solution with hydrochloric acid. Which indicator is the most appropriate for this titration?
Acid $K_{a}$
$H_{2} {CO}_{3}$ $2.5 \times 10^{- 4}$
${HCO}_{3}^{-}$ $2.5 \times 10^{- 8}$

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alizarin yellow $p K_{I n} = 12.5$

methyl orange, $p K_{I n} = 3.7$

bromothymol blue, $p K_{I n} = 7.0$

phenolphthalein, $p K_{I n} = 9.3$

answer is A.

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Detailed Solution

Methyl orange is most suitable. Methyl orange is a frequently used pH indicator in titrations because of its straightforward and obvious colour shift. Due to the fact that it changes colour at the pH of a moderate acid, it is frequently employed in acid titrations. Methyl orange changes colour from yellow in a basic medium to red in an acidic one.

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