Henderson-Hasselbach equation +

Question

1. Use the Henderson-Hasselbach equation to determine the ratio of acid to base in a formic acid-formate buffer with pH=3.00.

2. Assume that the human blood buffer includes, at any point, 0.00080 M carbonic acid and 0.0016 M hydrogen carbonate. Determine the number of moles of each component present in a person with 7.00 L of blood. What is the ratio of carbonic acid to hydrogen carbonate in this case?

3. Metabolic acidosis results in the addition of excess acid to blood. How many moles of acid must be added to the blood in Question 2 to bring the carbonic acid/hydrogen carbonate ratio to the hazardous level of 1:10?

4. Mixing acids and bases rapidly can be very dangerous. Before pouring any acid or base in a disposal unit, what can be done to neutralize the solution?

Answer 1

1.  HH equation is pH = pKa + log [salt]/[acid].  We need the pKa of formic acid. This is obtained from a table on the internet or in a chem. book.  It is 3.77.  Thus, 3.00 = 3.77 + log [salt]/[acid]

log salt/acid = 3.00 - 3.77 = -0.77

salt/acid = 0.171

So ratio of salt to acid is 0.171 : 1 but ...

The question asks for the ratio of acid/base or that's the same as the ratio of acid/salt as the salt is the conjugate base.  So

acid/salt = 1/0.171 = 5.85 : 1

You can check this by putting the value back in the original equation and solve for pH. 

pH = 3.77 + log 0.171 and pH = 3.00

 

2.  Carbonic acid is a weak acid and is CO(OH)2 also written as H2CO3. Hydrogen carbonate is the same as bicarbonate, the conjugate base of carbonic acid, and is HCO3-.

moles carbonic acid present = 0.00080 mole/L x 7.00 L = 0.0056 moles carbonic acid

moles hydrogen carbonate present = 0.0016 mol/L x 7.00 L = 0.0112 moles hydrogen carbonate

Ratio of carbonic acid to hydrogen carbonate = 0.0008/0.0016 = 0.5 or 0.0056/0.0112 = 0.5

NOTE:  The normal ratio of carbonic acid to hydrogen carbonate in human blood is 0.05 and not 0.5.

NOTE:  The normal hydrogen carbonate concentration in human blood is about 0.02 M, and not 0.0016 M

 

3.  Acid + hydrogen carbonate --> carbonic acid

H^+  +  HCO3^- ---> H2CO3

Since the ratio of carbonic acid to hydrogen carbonate is already 0.5, it cannot go to 0.1 (1:10) by the addition of acid. If the previous question meant to state that [HCO3-] was o.016, and not 0.0016, then this could be accomplished.

 

4.  The acids and bases can be diluted with water.  This is dilution, and not neutralization.  The only way to neutralize an acid is to add a base, and the only way to neutralize a base is to add an acid, but if they are diluted and mixed slowly, the danger decreases.

 

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