Alka-Seltzer is an over-the-counter medicine used to treat acid indigestion and

Question

Alka-Seltzer is an over-the-counter medicine used to treat acid indigestion and heartburn. The active pharmaceutical ingredients (API) in each Alka-Seltzer tablet are aspirin (325 mg), citric acid (1,000 mg) and sodium bicarbonate (1,916 mg). For a single dose, two Alka-Seltzer tablets are dissolved in 4.0 fluid ounces of water (1.0 fluid ounce = 29.57 mL), causing the following reaction:

C6H8O7 + 3NaHCO3 = 3H2O + 3CO2 + Na3C6H5O7

(a) What volume of CO2 gas (mL) at 25 degree C and 1 atm would be produced by a normal dose of Alka-Seltzer? Assume ideal-gas behavior and that the reaction goes to completion.

(b) You wake up feeling miserable before your material-and-energy-balances final exam, but you know you can't miss it. You grab some Alka-Seltzer and an 11-ounce bottle. You drink just enough water to leave 4.0 fluid ounces in the bottle. Then you drop in two tables, screw the cap tightly onto the bottle, and rush out of the door. Calculate the pressure inside the bottle, assuming the temperature remains at constant 25 degrees C and neglecting the volume of the tablets.

(c) How reasonable are the assumptions of the ideal-gas behavior in Parts(a) and (b)?

Explanation / Answer

a)

Volume of CO2 will be produced at T =? 25°C = 298K, P = 1 atm

for a normal dose:

singles dose --> 2 tablets...

mol of acid = mass/MW = 1/192.124 = 0.005204 mol of acid

then

1 mol of acid = 3 mol of CO2

so

0.005204 mol = 0.005204*3 = 0.015612 mol of CO2

PV = nRT

V = nRT/P

V = (0.015612)(0.082*298)/1 =0.381494Liters = 381.5 mL

b)

V = 11-4 = 7 oz

2 tables

mol of CO2 expected --> 0.015612 *2 = 0.031224 mol of CO2 in 2 tables

so

Volume of container left is acutally ---> 7 fluid oz = 207.015 mL

T = 25°C = 298K

so

PV = nRT

P = nRT/V

P = (0.031224 *0.082*298)/(207.015*10^-3) = 3.6856 atm

c)

in part a, is pretty likely, since T is medium, and P is low as well

in part b, there is also ok assumption, since P < 10 atm, so it is likely it will behave as ideal

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