Extended Response Section
Alka-Seltzer is a water-soluble medication that can treat acid indigestion in two ways; by neutralising
stomach acid using sodium hydrogencarbonate and treating the associated pain by using aspirin
(C9H8O4). When Alka-Seltzer was developed in the early part of the twentieth century, it was found
the solubility of the active ingredients were improved if a reaction that gives off a gas took place
when the tablet was added to water. Therefore, solid citric acid (C6H8O7) was added to the mixture
to react with some of the sodium hydrogencarbonate.
The reaction between sodium hydrogen carbonate and citric acid (C6H8O7) which causes the
+ 3H,O(l) + 3CO2(g)
The normal composition of one Alka-Seltzer tablet is:
Sodium hydrogen carbonate
Citric acid (C6H8O7)
In the stomach, the normal concentration of hydrochloric acid is 0.160 mol L-1. The volume
of the acid in the stomach can be assumed to be 0.650 L. A stressed student, studying
her Chemistry examination, eats a whole packet of biscuits, causing her concentration of acid
to rise to 0.200 mol -1.Calculate how many Alka-Seltzer tablets she should take to reduce
the level of acid back to normal.
If you were unable to calculate the concentration of sodium hydrogen carbonate in the Alka-
Seltzer tablet in part b i) above, then assume a final concentration of 6.00 X 10-2
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