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Limiting Reactants

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Limiting Reactants
Chemistry 101 Lab 7 Determination of % NaHCO3 in Alka Seltzer Tablets

Names ____________________________and ___________________________
Date ____________________________

Objectives:
(1) To determine the amount of NaHCO3 in Alka Seltzer tablets by observing the amount of CO2 produced from the acid-base reaction of HCO3- with acetic acid (in vinegar).

(2) To study the concept of limiting reactant.

Background:
Alka Seltzer is an effervescent tablet that contains aspirin (acetylsalicylic acid), citric acid, and sodium bicarbonate (NaHCO3). As soon as the tablet dissolves in water, the NaHCO3 dissociates to form a bicarbonate ion (HCO3-) and a sodium ion (Na+)

1. NaHCO3(s)  Na+(aq) + HCO3-(aq)
Acetic acid, which will ‘donate’ H+ to water to create H3O+(aq) is then added to the mixture. With the addition of acetic acid the following acid-base reaction then takes place:

2. HCO3-(aq) + H3O+(aq)  2H2O(l) + CO2 (g)

(The sodium ion does not take part in this reaction, and so is not included in the equation.) According to the reaction equation, one mole of HCO3- reacts with one mole of H3O+. If the number of moles of each reactant is not the same, then one reactant will be completely used up during the reaction while some of the other reactant will remain at the end of the reaction. The reactant that is completely used up is called the “limiting reactant”. In this experiment you will dissolve Alka Seltzer in various concentrations of vinegar and measure the amount of CO2 produced. From this data you will calculate the amount of NaHCO3 in the Alka Seltzer by percent weight. You will also determine the limiting reactant at each concentration.

Pre-lab Question:

a) What are the bubbles that form when Alka Seltzer is dissolved in water? Give the chemical formula.

b) What is the ratio of moles of CO2 produced to moles of NaHCO3 reacted?

Procedures:
1. Using a 50 mL graduated cylinder,

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