Bell Ringer Which are acid-base pairs according to the BrnstedLowry theory? I. HNO3/NO-3 II. H3O+/OHIII. HCOOH/HCOOA. I and II only B. I and III only C. II and III only D. I, II and III Bell Ringer
Which are acid-base pairs according to the BrnstedLowry theory? I. HNO3/NO-3 II. H3O+/OHIII. HCOOH/HCOO- Bell Ringer Which are acid-base pairs according to the BrnstedLowry theory? I. HNO3/NO-3
II. H3O+/OHIII. HCOOH/HCOOANSWER I and III only 1. Which gas in the atmosphere causes the pH of unpolluted rain to be approximately 6? A. Carbon dioxide B. Sulfur dioxide C. Oxygen D. Nitrogen Markscheme: A In Topic 8.5, it is explained on page 258 that ALL rainwater is naturally acidic due to
the presence of dissolved CO2 from the atmosphere, resulting in a pH of 5.6, which can be rounded to 6. When CO2 reacts with water it dissolves to form the weak acid carbonic acid, H2CO3 Acid rain has solutions that result in a pH BELOW 5.6 and therefore contain additional acids other than carbonic acid, such as sulfurous acid (H2SO3), sulfuric acid (H2SO4), nitrous acid (HNO2), and nitric acid (HNO3)! So unpolluted rainfall, despite being mildly acidic due to carbon dioxide, is still not considered to be acid rain. 2. Which compound is a strong acid? A. NH3
B. HNO3 C. H2CO3 D. CH3COOH Markscheme: B You already know from the last slide that nitric acid, HNO3, is a contributor to acid rain, which is very acidic. You also already know from the last slide that carbonic acid, H2CO3, is a weak acid that is created from rainfall and carbon dioxide. CH3COOH is ethanoic acid, or acetic acid, which is found in vinegar, which means it is WEAK.
3. 10.0 cm3 of a solution of a strong acid with a pH of 3 is added to a volumetric flask and the total volume is made up to 1.00 dm3 by adding distilled water. The resulting solution is then thoroughly mixed. What is the pH of the diluted solution? A. 1 B. 2 C. 4 D. 5 Markscheme: D This solution was diluted by 1.00 dm3, which is the same as saying 1,000 cm3.
Since it went from being 10.0 cm3 to being 1,000 cm3 this means the solution has changed by a factor of100, or 10 x 10, which we know is equivalent to two units on the pH scale since each unit change on the pH scale is 10. The pH has therefore INCREASED by 2 since this was a dilution (which causes hydrogen ion concentration to decrease), which means it has gone from 3 to 5. 4. Was the Bell Ringer 5. A solution of 50cm3 hydrochloric acid has a pH of 4. What is the final pH if 450cm3 of water is added? A. 3 B. 4
C. 5 D. 6 Markscheme: C If 450cm3 is added to 50cm3, then the resulting HCl has been diluted by a factor of roughly 10 (450/50 = 9). This means that the pH has become more basic by a factor of 1, meaning pH has gone from 4 to 5. 6. Which compound reacts with calcium oxide, CaO? A. K2O B. Na2O C. SO2
D. MgO Markscheme: C Examiners report This question assumed an understanding of Periodicity (3.3.2) and Acid-Base (8.2.1). The majority of candidates (69%) had no difficulty in determining that calcium oxide is a basic oxide and choosing the only acidic oxide, sulfur dioxide, as the answer. 7. A student adds 0.3 g of magnesium metal to equal volumes of hydrochloric acid and ethanoic acid of the same concentrations in separate flasks. Which statement is correct?
A. Hydrochloric acid reacts more rapidly as it has a higher pH than ethanoic acid. B. A greater total volume of H2 gas is produced with hydrochloric acid than with ethanoic acid. C. The same total volume of H2 gas is produced with both hydrochloric acid and ethanoic acid. D. Ethanoic acid reacts more slowly because it has a lower pH than hydrochloric acid. Markscheme: C 8.
9. Markscheme: B 10. A solution of acid HX has a pH = 1 and a solution of acid HY has a pH = 3. Which statement must be correct? A. HX is a stronger acid than HY. B. HY is a stronger acid than HX. C. The [H+] in the solution of HX is 100 times greater than the [H+] in the solution of HY.
D. The [H+] in the solution of HY is 100 times greater than the [H +] in the solution of HX. Markscheme: C DO NOT FORGET! STRENGTH AND CONCENTRATION ARE NOT NECESSARILY RELATED TO pH! Strength is related to dissociation/ability to ionize. pH is associated to concentration of H+
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