| Date | May 2017 | Marks available | 1 | Reference code | 17M.3.sl.TZ1.20 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | Calculate | Question number | 20 | Adapted from | N/A |
Question
The buffer formed by carbon dioxide, CO2(aq) and hydrogen carbonate ion, HCO3−(aq), plays an important role in maintaining the pH of blood.
Calculate the pH of the buffer from the following data and section 1 of the data booklet.
pKa(CO2) = 6.34
[HCO3−(aq)] = 1.40 × 10−2 moldm−3
[CO2(aq)] = 1.25 × 10−3 moldm−3
Explain the effect of a large amount of aspirin on the pH of blood.
Markscheme
«pH = pKa + log10 » = 7.39
[1 mark]
H+ from aspirin reacts with HCO3− to form CO2 and H2O
OR
H+(aq) + HCO3−(aq) CO2(aq) + H2O(l)
OR
reverse reaction favoured «to use up some of the H+ added»
pH decreases
No mark for “stating aspirin is a weak acid that dissociates partially to produce H+” without reference to reaction with HCO3− or to the equation.
Reversible arrows not required for the mark.
Do not accept “small pH change when small amount of H+ is added”.
[2 marks]
Examiners report
Syllabus sections
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16N.3.sl.TZ0.19a:
Ranitidine (Zantac) is a drug that inhibits stomach acid production. Outline why the development of this drug was based on a detailed knowledge of the structure of histamine, shown below.
- 18N.3.sl.TZ0.14c: Suggest a technique for measuring the percentage mass of calcium carbonate in this type of...
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17N.3.sl.TZ0.20b:
The pH is maintained in different fluids in the body by the use of buffers.
Calculate the pH of a buffer solution of 0.0200 mol dm–3 carbonic acid, H2CO3, and 0.400 mol dm–3 sodium hydrogen carbonate, NaHCO3. The pKa of carbonic acid is 6.35.
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19M.3.hl.TZ2.23a(i):
Outline how ranitidine (Zantac) inhibits stomach acid production.
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17M.3.sl.TZ2.18a.i:
Compare and contrast the structures of oseltamivir and zanamivir, stating the names of functional groups.
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17N.3.sl.TZ0.20a:
Explain how ranitidine (Zantac) reduces stomach acid production.
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16N.3.sl.TZ0.19b:
Two other drugs, omeprazole (Prilosec) and esomeprazole (Nexium), directly prevent the release of acid into the stomach. Identify the site of action in the body.
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17M.3.hl.TZ2.22a:
Outline how ranitidine (Zantac) functions to reduce stomach acidity.
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18M.3.sl.TZ2.18b:
Calculate the amount, in mol, of stomach acid neutralized by an antacid tablet containing 0.750 g calcium carbonate.
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19N.3.sl.TZ0.16a:
Identify the compound responsible for the acidity of gastric juice, and state whether it is a strong or weak acid.
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18M.3.sl.TZ1.14b:
Explain how omeprazole (Prilosec) reduces stomach acidity.
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18N.3.sl.TZ0.14a:
Determine the pH of a buffer solution that is 0.0100 mol dm−3 sodium hydrogen carbonate and 0.0200 mol dm−3 sodium carbonate, using section 1 of the data booklet.
Ka (hydrogen carbonate ion) = 4.8 × 10−11
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17M.3.sl.TZ2.17a:
Two drugs are ranitidine (Zantac) and omeprazole (Prilosec). Outline how they function to reduce stomach acidity.
- 19N.3.sl.TZ0.16c: Outline how ranitidine reduces stomach acidity.
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19M.3.sl.TZ2.16a(ii):
Outline two advantages of taking ranitidine instead of an antacid which neutralizes excess acid.
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19N.3.sl.TZ0.16d:
Calculate the pH of a buffer solution which contains 0.20 mol dm−3 ethanoic acid and 0.50 mol dm−3 sodium ethanoate. Use section 1 of the data booklet.
pKa (ethanoic acid) = 4.76
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16N.3.sl.TZ0.19c:
A different approach to treating excess stomach acid is to neutralize it with antacids. Formulate an equation that shows the action of an antacid that can neutralize three moles of hydrogen ions, H+, per mole of antacid.
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19M.3.sl.TZ1.16a(ii):
Determine the volume of CO2 (g), in dm3, produced at STP, when 1.00 g of CaCO3 (s) reacts completely with stomach acid.
Mr CaCO3 = 100.09
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19M.3.sl.TZ2.16b:
Some antacids contain carbonates.
Determine the pH of a buffer solution which contains 0.160 mol dm−3 CO32− and 0.200 mol dm−3 HCO3−, using section 1 of the data booklet.
pKa (HCO3−) = 10.32
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18N.3.sl.TZ0.14b:
State the equation for the reaction of calcium carbonate, the active ingredient in some antacids, with stomach acid.
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18M.3.sl.TZ2.18a:
Formulate a chemical equation for the neutralization of stomach acid with calcium carbonate.
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18M.3.sl.TZ1.14a.ii:
Determine the amount, in mol, of hydrochloric acid neutralized by one antacid tablet.
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19N.3.sl.TZ0.16b:
An antacid contains calcium carbonate and magnesium carbonate.
Write the equation for the reaction of magnesium carbonate with excess stomach acid.
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19M.3.hl.TZ1.21a:
Formulate an equation for the neutralization of stomach acid with calcium carbonate, CaCO3 (s).
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17M.3.sl.TZ2.17b:
0.500 g of solid anhydrous sodium carbonate, Na2CO3(s), is dissolved in 75.0 cm3 of 0.100 moldm−3 sodium hydrogen carbonate solution, NaHCO3(aq). Assume the volume does not change when the salt dissolves.
HCO3−(aq) CO32−(aq) + H+(aq) pKa = 10.35.
Calculate the pH of the buffer solution.
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19M.3.hl.TZ2.23a(ii):
Outline two advantages of taking ranitidine instead of an antacid which neutralizes excess acid.
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19M.3.hl.TZ2.23b:
Some antacids contain carbonates.
Determine the pH of a buffer solution which contains 0.160 mol dm−3 CO32− and 0.200 mol dm−3 HCO3−, using section 1 of the data booklet.
pKa (HCO3−) = 10.32
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18M.3.sl.TZ2.18c:
Explain how omeprazole (Prilosec) regulates pH in the stomach.
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19M.3.sl.TZ1.16a(i):
Formulate an equation for the neutralization of stomach acid with calcium carbonate, CaCO3 (s).
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17M.3.sl.TZ2.18a.ii:
Deduce the wavenumber of one absorbance seen in the IR spectrum of only one of the compounds, using section 26 of the data booklet.
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18M.3.sl.TZ1.14a.i:
An antacid tablet contains 680 mg of calcium carbonate, CaCO3, and 80 mg of magnesium carbonate, MgCO3.
State the equation for the reaction of magnesium carbonate with hydrochloric acid.
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17M.3.sl.TZ1.20b:
Explain the effect of a large amount of aspirin on the pH of blood.
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19M.3.hl.TZ1.21b:
Acid secretion can be regulated by other types of drugs such as omeprazole and ranitidine. Outline how each of these drugs acts to reduce excess stomach acid.
Omeprazole:
Ranitidine:
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19M.3.sl.TZ2.16a(i):
Outline how ranitidine (Zantac) inhibits stomach acid production.
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19M.3.sl.TZ1.16b:
Acid secretion can be regulated by other types of drugs such as omeprazole and ranitidine. Outline how each of these drugs acts to reduce excess stomach acid.
Omeprazole:
Ranitidine:
