Question

Given three acid-base indicators-methyl orange (end point at \(\mathrm{pH} 4\) ), bromthymol blue (end point at \(\mathrm{pH} 7\) ), and phenolphthalein (end point at \(\mathrm{pH} 9\) ) - which would you select for the following acid- base titrations? (a) perchloric acid with an aqueous solution of ammonia (b) nitrous acid with lithium hydroxide (c) hydrobromic acid with strontium hydroxide (d) sodium fluoride with nitric acid

Short answer

Question: Select the appropriate acid-base indicator for the following acid-base titrations. (a) Perchloric acid with an aqueous solution of ammonia. (b) Nitrous acid with lithium hydroxide. (c) Hydrobromic acid with strontium hydroxide. (d) Sodium fluoride with nitric acid. Answer: (a) Methyl orange (Endpoint pH: 4) (b) Phenolphthalein (Endpoint pH: 9) (c) Bromthymol blue (Endpoint pH: 7) (d) Methyl orange (Endpoint pH: 4)

Step-by-step solution

(a) Perchloric acid with an aqueous solution of ammonia

For this titration, we have a strong acid (perchloric acid) reacting with a weak base (ammonia). The reaction can be represented as: $$\mathrm{HClO_{4}(aq) + NH_{3}(aq) \rightarrow ClO_{4}^{-}(aq) + NH_{4}^{+}(aq)}$$ At the equivalence point, the moles of strong acid should be equal to the moles of weak base. Since the strong acid is in control, the pH at the equivalence point should be less than 7. We can then choose methyl orange as the appropriate indicator for this titration, as its endpoint pH is 4, which is closest to the expected pH at the equivalence point for this scenario.

(b) Nitrous acid with lithium hydroxide

In this case, we have a weak acid (nitrous acid) reacting with a strong base (lithium hydroxide). The equation for the reaction is: $$\mathrm{HNO_{2}(aq) + LiOH(aq) \rightarrow NO_{2}^{-}(aq) + Li^{+}(aq) + H_{2}O(l)}$$ At the equivalence point, the moles of weak acid should be equal to the moles of strong base. In this case, the strong base is in control, so the pH at the equivalence point should be greater than 7. We can then choose phenolphthalein as the appropriate indicator for this titration, as its endpoint pH is 9, which is closest to the expected pH at the equivalence point for this scenario.

(c) Hydrobromic acid with strontium hydroxide

Here, we have a strong acid (hydrobromic acid) reacting with a strong base (strontium hydroxide). The reaction can be written as: $$\mathrm{2HBr(aq) + Sr(OH)_{2}(aq) \rightarrow SrBr_{2}(aq) + 2H_{2}O(l)}$$ At the equivalence point, the moles of strong acid should be equal to the moles of strong base. In this case, the pH at the equivalence point should be 7, as neither the acid nor the base is in control. We can then choose bromthymol blue as the indicator for this titration since its endpoint pH is 7, matching the expected pH at the equivalence point.

(d) Sodium fluoride with nitric acid

In this titration, we have a weak base (sodium fluoride, or NaF) reacting with a strong acid (nitric acid). The reaction can be represented as: $$\mathrm{NaF(aq) + HNO_{3}(aq) \rightarrow NaNO_{3}(aq) + HF(aq)}$$ At the equivalence point, the moles of weak base should be equal to the moles of strong acid. Since the strong acid is in control, the pH at the equivalence point should be less than 7. We can then choose methyl orange as the appropriate indicator for this titration, as its endpoint pH is 4, which is closest to the expected pH at the equivalence point for this scenario.