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, add 35 mL of distilled water to a clean 250 mL beaker.
Weigh and record the total mass of the beaker plus water (column 1,Run #1, in data table). Also record the measured volume of the water (don’t just write 35 mL). 2. Weigh and record the mass of an Alka Seltzer tablet (with correct sig. figs. ) 3. Drop the tablet into the beaker and swirl carefully. As soon as the bubbling ceases, weigh the beaker with the dissolved Alka Seltzer solution. Record this mass (with correct sig. figs. ) 4. Rinse the beaker with distilled water. Then, repeat steps 1-3 with 5 mL vinegar + 30 mL water, 10 mL vinegar + 25 mL water, and so on, increasing the volume of vinegar by 5 mL each time while keeping the total volume of the solution at 35 mL.
Record your data in the table below (Run #2 – Run #8). Again, be sure to record the actual measured volumes and record them with the correct number of significant figures for the glassware that you use. Data: Run #1 Run #2 Run #3 Run #4 Run #5 Run #6 Run #7 Run #8 Volume of vinegar (mL) 0. 00 Volume of water (mL) 0. 00 Mass of beaker with liquid (g) Mass of Alka Seltzer tablet (g) Mass of beaker with liquid after bubbles cease (g) Calculated mass of CO2 lost (g) Moles of CO2 lost Moles of NaHCO reacted Calculated mass of NaHCO3 reacted (g) Calculated % by mass of reacted NaHCO3 in tablet Analysis: 1.
Do the calculations in the data table (in bold). Show work for an example of each type of calculation (use run #1) below. Calculations for run #1: 2. Plot the calculated percent by mass of the reacted NaHCO3 in a tablet vs. the volume of vinegar used, either by hand (on graph paper) or using a computer spreadsheet. . Post-Lab Questions: 1. Based on your plot, at what range of volumes of vinegar is HCO3- the limiting reactant? (hint: determine when you had enough vinegar to react will all of the HCO3-) 2. Based on your data and your plot, what is the percent by mass of NaHCO3 in the average Alka Seltzer tablet?