Part I
Calculations:
Calculate the hydrogen ion concentration, [H+ ], for the two weak acids (pH=-log[H+ ], or [H+ ]=antilog (-pH). If you have difficulty finding or using the antilog function on your calculator, simply use this: [H+ ]=10-pH . Record these in your data table.
Calculate the hydroxide ion concentration, [OH- ], for the weak base using this formula: pOH=14-pH, then [OH- ]=antilog (-pOH) or [OH- ]=10-pOH. Record this in your data table.
Record [A- ] or [BH+ ] in your data table. Refer to the information provided in the introductory section of this lab.
The concentration of carbonic acid, which is the seltzer water, can be found in a handbook of chemistry and physics and is 0.0341 M (at STP). Record this in your data table.
You will need to calculate the molar concentrations of the vinegar as well as ammonia. Both are industry standard 5.00% by mass solutions in water. This means 5.00 g of solute/ 100.00 g of solution. Assume that the solution is mostly water (which it is) and thus has a density of 1.00 g H2O/1.00 ml H2O. Now use this information plus the molar masses of the vinegar (acetic acid) and ammonia (ammonium hydroxide) to change the 5.00 g solute/100.00 g solution to M, which is mol solute/L solution (H2O). Record these concentrations in your data table.
Calculate the Ka or Kb using the formula given in the introduction. Record these in your data table.
Calculate the pKa or pKb (pKa/b =-log Ka/b) and record these in your data table.
Discussion Questions:
Find the Ka/b values in Appendix D tables 1 (Ka1 only) and 2 and calculate the pKa/b values.
Calculate your percentage error.
Explain mathematically and chemically why you cannot measure or calculate a pKa or pKb for a strong acid or strong base.
Part II
Discussion
Salts can be formed by the reaction of an acid with a base. For each of the four salts you tested, write the reaction of an acid with a base to form that salt. Indicate whether the acid and base are strong or weak.
Write the dissociation equations for each of the four salts (into ions- you should use the subscript (aq) for them).
Write the hydrolysis equations for each of the four (4) salts (see intro for examples).
According to your data, indicate which salts appear to be neutral, acidic, and alkaline.
Explain why each salt appears to be neutral, acidic, or alkaline.
List an example other than the salts used in this lab for each of the following:
a neutral salt (formed by the reaction of a strong acid with a strong base)
a neutral salt (formed by the reaction of a weak acid with a weak base)
an acidic salt
an alkaline salt
In the lab, which of the salts could be used to make a buffer and why?
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