Sodium carbonate (Na_2CO_3), also known as washing soda, is the sodium salt of c

Question

Sodium carbonate (Na_2CO_3), also known as washing soda, is the sodium salt of carbonic acid. It is caustic and a relatively strong base with a variety of uses. It is used in making glass, to adjust the pH of swimming pools, and to remove flesh in taxidermy, among other things. Joe the Plumber uses washing soda to remove oil and grease from his tools. Being responsible and safety conscious he wants to neutralize this base with an acid before disposing of his waste solutions. He uses acidic vinegar for neutralization. Washing soda is sodium carbonate, Na_2CO_3, which dissolves and dissociates in water. One molecule of sodium carbonate produces two sodium ions and a basic carbonate ion in solution. The active ingredient in vinegar is a acid in bold). One carbonate ion uses acidic protons from two a acid molecules to form one carbonic acid molecule (H_2CO_3), which decomposes into carbon dioxide and water. The chemistry of this neutralization process can be broken down into four steps: (1) loss of the acidic proton by acetic acid, (2) gain of a proton by a carbonate ion to produce hydrogen carbonate, (3) gain of a second proton by hydrogen carbonate to produce carbonic acid, and (4) finally decomposition of carbonic acid in a gas evolution reaction. 2 CH_3COOH _(aq) rightarrow 2 CH_3COO (aq) + 2 H^+_(aq) H^+_(aq) + CO_3^2_(aq) rightarrow HCO_3(aq) H^+(aq) + HCO_3_(aq) rightarrow H_2CO_3_(aq) H_2CO_3_(aq) rightarrow H_2O_(t) + CO_2(g) Reactions 1 through 3 can be added together to give the overall neutralization reaction: (5)_________________ Reactions 4 and 5, when added together, give a new overall gas evolution reaction: (6) ________________________________ Joe the Plumber dissolves 54 g of washing soda in water to make 250 mL of his degreasing solution. Determine the minimum volume of vinegar he needs to neutralize one liter of his degreasing solution. Assume that vinegar has a density of 1.01 g/mL and contains 5% acetic acid by mass. Before starting calculations, try devising a written step-by-step solution strategy based on what you know and what you want to find. Activity adapted from: Solving Real Problems with Chemistry, 2^nd Edition. (2012) Goodwin, J., Hanson, D., and, T.(Editors). Pacific Crest, Plainfield, IL 148 pp.

Explanation / Answer

Sodium carbonate (Soda ash) is the second most economical neutralization agent next to Lime.

Maximum pH is approximately 11 at room temperature (25 oC)

Neutralization reaction of Sodium carbonate using acetic acid

1. 2CH3COOH 2CH3COO- + 2H+

2. H+ + CO32- HCO3-

3. H+ + HCO3- H2CO3

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5. 2CH3COOH + CO32- 2CH3COO- + H2CO3 by canceling the common ions which have been underlined on the both the side of arrows in all the three reactions we got 5th reaction.

4. H2CO3 H2O + CO2

5. 2CH3COOH + CO32- 2CH3COO- + H2CO3

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6. 2 CH3COOH + CO32- 2CH3COO- + H2O + CO2 by canceling the common ions which have been underlined on the both the side of arrows in all the three reactions we got 6th reaction

Both the reactants have to be dissolved in water so that their ions can then interchange. Two Na+ cations leave to negatively charged of acetate and bond to it, form the molecule of sodium carbonate. Two Protons H+ is attracted from two molecules of acetic acid towards CO32- to give H2CO3. Which further decomposed to a molecule of water and a molecule of CO2 which escapes from the reaction with the formation fo effervescence.

Molar mass of Na2CO3 = 106 g mol-1

First, calculate the concentration of the degreasing solution. Convert the given weight (54 g) of Na2CO3 into no. of moles. For that, formula is

No. of moles = Weight of the substance / Molecular weight of the substance

No. of moles = 54 g / 106 = 0.5094 moles present in the 250 mL of water. Even after further dilution of the solution, the no of moles will be constant. Now we know the moles and we have to convert it into molarity. Formula is,

Molarity = No of moles / Volume in liters

Molarity = 0.5094 mol / 0.250 = 2.0376 M is the concentration of the Joe the plumbers degreasing solution. one liter of his degreasing solution is also of 2.0376 M concentration. For neutralization of 2.0376 M solution of Sodium carbonate stoichiometric amount of acetic acid is required.

The vinegar is a 5.0% solution of acetic acid Molecular weight 60.05 g/mole, 5 g acetic acid 100 g solution with density 1.01 g/mL, Now determine the molarity of vinegar.

Convert grams of acetic acid into moles acetic acid and gram of solution into mL solution and then to Liter

5 g acetic acid X 1.01 g/mole X 1 mole acetic acid X 1000 mL 100 g solution 1 mL Solution 60.05 g acetic acid 1 L

0.84 Mole per liter

Molarity = No of moles / Volume in liters

Molarity = 0.84 mol / 0.250 = 3.36 M

One liter of his degreasing solution has 2.0376 M concentration One liter of 5% acetic acid has 3.36 M concentration

2.0376 X1000 / 3.36 = 607 mL

607 mL of 5 % acetic acid is required for neutralizing the degreasing solution because 607 mL of acetic acid solution contains 2.0376 M concentration of acetic aicd.

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