Acid And Bases Calculations Practice Worksheet

[Fermina Enge]

Manyacids and bases are weak; that is, they do not ionize fully in aqueous solution. A solution of a weak acid in water is a mixture of the nonionized acid, hydronium ion, and the conjugate base of the acid, with the nonionized acid present in the greatest concentration. Thus, a weak acid increases the hydronium ion concentration in an aqueous solution (but not as much as the same amount of a strong acid).

giving an equilibrium mixture with most of the acid present in the nonionized (molecular) form. This equilibrium, like other equilibria, is dynamic; acetic acid molecules donate hydrogen ions to water molecules and form hydronium ions and acetate ions at the same rate that hydronium ions donate hydrogen ions to acetate ions to reform acetic acid molecules and water molecules. We can tell by measuring the pH of an aqueous solution of known concentration that only a fraction of the weak acid is ionized at any moment (Figure \(\PageIndex1\)). The remaining weak acid is present in the nonionized form.

Table \(\PageIndex1\) gives the ionization constants for several weak acids. Acid ionization constants can be determined by measuring the equilibrium concentrations of a weak acid and its conjugate base in solution.

Acetic acid is the principal ingredient in vinegar; that's why it tastes sour. At equilibrium, a solution contains [CH3CO2H] = 0.0787 M and \(\ce[H3O+]=\ce[CH3CO2- ]=0.00118\:M\) . What is the value of Ka for acetic acid?

The situation for basic solutions is similar. At equilibrium, a solution of a weak base in water is a mixture of the nonionized base, the conjugate acid of the weak base, and hydroxide ion with the nonionized base present in the greatest concentration.

We can confirm by measuring the pH of an aqueous solution of a weak base of known concentration that only a fraction of the base reacts with water (Figure \(\PageIndex3\)). The remaining weak base is present as the unreacted form. The equilibrium constant for the ionization of a weak base, Kb, is called the ionization constant of the weak base, and is equal to the reaction quotient when the reaction is at equilibrium. For trimethylamine, at equilibrium:

In practice, chemists almost never measure the equilibrium concentrations of both the weak acid and conjugate base forms, but rather determine them from the known total acid concentration and the measured pH. Remember that pH gives the concentration of \(\ceH3O+\) in a solution - this quantity can then be used in the acid dissociation equilibrium. For weak bases, pH can be converted to pOH to relate to the concentration of \(\ceOH-\) in the base dissociation equilibrium. Conversely, if the Ka or Kb is known, the pH can be predicted from the total acid/base concentration using the same calculational approach.

We can summarize the various concentrations and changes as shown here (the concentration of water does not appear in the expression for the equilibrium constant, so we do not need to consider its concentration):

Solution This problem requires that we calculate an equilibrium concentration by determining concentration changes as the ionization of a base goes to equilibrium. The solution is approached in the same way as that for the ionization of formic acid in Example \(\PageIndex6\). The reactants and products will be different and the numbers will be different, but the logic will be the same:

Some weak acids and weak bases ionize to such an extent that the simplifying assumption that x is small relative to the initial concentration of the acid or base is inappropriate. As we solve for the equilibrium concentrations in such cases, we will see that we cannot neglect the change in the initial concentration of the acid or base, and we must solve the equilibrium equations by using the quadratic equation.

We need to determine the equilibrium concentration of the hydronium ion that results from the ionization of \(\ceHSO4-\) so that we can use \(\ce[H3O+]\) to determine the pH. As in the previous examples, we can approach the solution by the following steps:

As we begin solving for x, we will find this is more complicated than in previous examples. As we discuss these complications we should not lose track of the fact that it is still the purpose of this step to determine the value of x.

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This acid and base calculations practice worksheet assess students on their ability to calculate for pH, pOH, [H]+, and [OH]-. There are 6 parts to this worksheet, each consisting of 5 calculation questions for a total of 30 questions.

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pH , p[H] and "pH" values iteratively computed with an accurate general equation (instead of the aproximate Henderson-Hasselbalch equation) combined with activity coefficients estimation by the Davies equation;

provides buffer capacity (buffer index, buffer strength), ionic strength, fractional distribution, medium charge of HiB to find the Isoelectric Point of amino acids , activities and apparent dissociation constants of all species at equilibrium;

the Regression spreadsheet assists the modeling of titrarion curves for the determination of concentrations of acids, salts and bases in the sample and the refinement of pKa values by least squares nonlinear regression essential for titrations of very diluted and/or complex samples that generate curves with ill-ndefined inflections, eg., acid rain. This advanced feature takes into account the effects of ionic strength and activity coefficients.

Titration module for beginners with clickable digital burette, color transition of the chosen indicator for visual endpoint detection and generation of samples with random unknown concentration;

composition diagrams (alpha plots, distribution curves or molar fraction curves) and buffer capacity curves of any mono or multiprotic acid or base showing the fractional contribution of each protonated and unprotonated species in equilibrium;

distribution curves plotted against volume of titrant, with overlaid titration curve, revealing the principal species at the inflections (endpoints) and the contribution of each species at any stage of the titration;

protonation curves an average charge curves of acids or bases showing the average number of protons bound to the Brnsted-Lowry (or Lewis) base and their weighted charges as a function of pH as well as volume of titrant (with overlaid titration curve); a zero-crossing of the charge reveals an Isoelectric Point.

CurTiPot is released as freeware for personal, educational and non-commercial use; for other applications, contact the author (copyright holder). Download the most recent release of CurTiPot, free of virus, spyware and addware, from this page .

There is no need to install (or uninstall) CurTiPot. Simply run the Microsoft ExcelTM software and open the curtipot_.xlsm file like any other spreadsheet workbook. All preprogrammed equations and Visual Basic embedded macros are self-contained and completely removed when closing CurTiPot (no functions added, no changes made to Excel).

It is necessary to enable macros in Excel to activate the buttons that run the macros for iterative computing of pH, distribution curves generation, smoothing, etc. Follow the instruction given in the Download section or, for older MS Windows versions, read instructions on the front page of CurTiPot, cell AM12 or refer to Microsoft.

All worksheets have embedded instructions and comments, editable/expandable by the user. In addition, the CurTiPot option i version contains balloons (bubble text) with a numbered sequence of first steps to be followed by beginners, like a recipe. Balloons can be deleted as they become superfluous.

The Regression module of CurTiPot uses an Excel supplement named Solver for the determination of concentrations and pKas of acids and bases from titration data by nonlinear least squares regression. If not yet visible in the Excel Data tab, Solver needs to be activated by checking the box in the Excel Supplements list.

6 Conductimetric titration data treatment with CurTiPot-Cond - Tratamento de dados de titulao condutomtrica com CurTiPot-Cond* (5:06)

* A copy of the beta version of CurTiPot-Cond can be requested from the author by the e-mail gu...@iq.usp.br

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