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Unit 1 Water - Buffers
1. Solubility of substances in water Predict which of the following substances are soluble in water. Explain you answer. Answer: Only Vitamin C is soluble in water as it contains multiple hydroxyl group that can contribute to hydrogen bonding. Vitamin A and β-carotene contain large and cyclic structure, which is hydrophobic, so they are insoluble in water. 2. Calculation of pH of pure water The ion product of water at 0°C is 1.14 × 10 – M^2 , and at 100°C it is about 5.13 × 10 –13^ M^2. What is the pH of pure water at 0°C and 100°C? Answer: At 0oC, Kw = 1.14 x 10-15, so: [H+] = 3.37 x 10-8^ M pH = - log [H +] = 7, At 100oC, Kw =5.13 × 10 –13^ M^2 , so: [H+] = 7.16 x 10 -7^ M pH = - log [H +] = 6,
3. Identifying the Conjugate Base Which is the conjugate base in each of the pairs below? (a) RCOOH, RCOO- (b) RNH 2 , RNH 3 + (c) H 2 PO 4 -^ , H 3 PO 4 (d) HPO 4 2-^ , H 2 PO 4 -^ - (e) PO 4 3-^ , HPO 4 2- (f) H 2 CO 3 , HCO 3 - (g) HCO 3 - , CO 3 2- Answer a) RCOO- b) RNH 2 c) H 2 PO 4 - d) HPO 4 2- e) PO 4 3- f) HCO 3 - g) CO 3 2- 4. Acidity of Gastric HCl In a hospital laboratory, a 10.0 mL sample of gastric juice, obtained several hours after a meal, was titrated with 0.1 M NaOH to neutrality; 7.2 mL of NaOH was required. The patient’s stomach contained no ingested food or drink; thus assume that no buffers were present. What was the pH of the gastric juice? Answer: HCl: VHCl = 10 mL, MHCl =? NaOH: VNaOH = 7.2 mL, MNaOH = 0.1 M NaOH + HCl NaCl + H 2 O nNaOH = 7.2 x 0.1= 0.72 mol nHCl = nNaOH = 0.72 mol MHCl = 0. 10
= 0.072M = [H+]
pH = -log[H+] = 1.
It is absorbed into the blood through the cells lining the stomach and the small intestine. Absorption requires passage through the plasma membrane, the rate of which is determined by the polarity of the molecule: charged and highly polar molecules pass slowly, whereas neutral hydrophobic ones pass rapidly. The pH of the stomach contents is about 1.5, and the pH of the contents of the small intestine is about 6. Is more aspirin absorbed into the bloodstream from the stomach or from the small intestine? Clearly justify your choice Answer: Aspirin is in its protonated (neutral) form at a pH lower than 3.5 and has a pKa of 3.5. Aspirin gets more deprotonated (anionic) when the pH rises. This is because passing through the plasma membrane is necessary for absorption. Now that the polarity of the molecule affects the pace of this absorption, charged molecules will move slowly while neutral hydrophobic molecules will move quickly. As a result, aspirin is more effectively absorbed in the stomach's acidic environment.
