Chemistry

Name: ___________________________ ACID-BASE TITRATIONS

A. Titration 

The progressive addition of one reagent (chemical reactant) to another. An acid-base titration can involve the addition of an acid to a base or vice-versa.

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Usually a solution of known concentration (the _________) is placed in a buret and is progressively added to a measured volume of another solution of unknown concentration (the __________).

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The titrant is added until the reaction mixture reaches the equivalence point

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equivalence point  point where chemically equivalent amounts of acid and base have reacted (& neither is in excess). The acid-base reaction is complete at this point.  equivalent = the amount of acid or base that gives 1 mol of H+ or OH- ions, respectively  Ideally, this is the same point at which an indicator changes colour = endpoint = pH (the endpoint is the experimental estimate of the equivalence point)  The volume of titrant added to achieve the equivalence point can be used to calculate the concentration of the sample via stoichiometry calculations

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Chemistry

Name: ___________________________

Primary Standards 

HCl and NaOH are the most commonly used titrants in acid-base titrations. However, neither is a primary standard : a solution that can be prepared from a pure form of the solute to a precise molar concentration

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Solutions of HCl and NaOH have to be standardized. A _________________ solution is one whose concentration is determined from data collected by titration with a primary standard solution

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HCl has to be standardized because it is a gas at room temperature. It is difficult to dissolve precise amounts of any gas in water. Moreover, once dissolved, tiny amounts of HCl have the tendency to escape from solution thereby reducing the concentration of dissolved HCl.

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NaOH is a deliquescent or hygroscopic solid - it absorbs ________________ from the air. When you weigh out a sample of NaOH, you are obtaining the mass of NaOH plus the mass of the water it has absorbed from the air. If you prepare a solution using the measured mass of NaOH, its molar concentration will be slightly lower than expected because of the absorbed moisture.

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Acids like HCl are standardized using a primary standard such as sodium carbonate (Na2CO3), while bases like NaOH are standardized using potassium hydrogen sulfate (KHSO4) or potassium hydrogen phthalate (KHC8H4O4).

Exercise: 1. If 21.56 mL of NaOH solution is required to neutralize 25.00 mL of a 0.0521 M standard oxalic acid solution, calculate the [NaOH].

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Chemistry

Name: ___________________________

ACID-BASE STOICHIOMETRY 

Can be used to predict the concentration of an unknown in a titration (most common application) or to find the volume of acid or base needed for a titration.

 Steps for solving problems: 1. Write a balanced chemical equation (double replacement & nonionic), then identify and record the required and given substances. 2.

If not given moles, convert the given quantity (mass, or solution volume and concentration) to number of moles using one of the formulas below: n = m/M

3.

n = Cv

Use the mole ratio to calculate the moles of the required substance from the moles of the given substance. (Every problem will involve this calculation). nR = nG x R/G

where nR = moles of required substance nG = moles of given substance

R/G = mole ratio using the coefficients from the balanced equation ie. moles of required/ moles of given 4.

If necessary, convert the moles of the required substance to the required quantity (mass, solution volume or concentration) using one of the formulas m = nM

v = n/C

C = n/v

Example 1: (to find unknown concentration) Determine the concentration of NaOH(aq) if 25 mL of NaOH is neutralized by 50 mL of 2.0 mol/L HCl(aq).

Example 2: (to find unknown volume) Calculate the volume of 0.50 mol/l H2SO4(aq) required to completely neutralize 40 mL of 0.90 mol/L LiOH(aq).

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