Solved Questions on Acids and Bases
1. You have two solutions, Solution A with a pH of 3.0 and Solution B with a pH of 5.0. Calculate the ratio of the hydrogen ion concentrations ([H+]) in Solution A to Solution B. Which solution is more acidic?
a) 100:1, Solution A is more acidic.
b) 1:100, Solution B is more acidic.
c) 10:1, Solution A is more acidic.
d) 1:10, Solution B is more acidic.
Answer: c) The pH scale is a logarithmic scale, and the relationship between pH and hydrogen ion concentration ([H+]) is as follows:
pH = -log10[H+]
To calculate the ratio of [H+] in Solution A to Solution B, we can first calculate the hydrogen ion concentrations for both solutions and then compare them.
For Solution A (pH = 3.0):
[H+] Solution A = 10-pH = 10-3 = 0.001 M
For Solution B (pH = 5.0):
[H+] Solution B = 10-pH = 10-5 = 0.00001 M
Now, let's compare the two concentrations:
[H+] Solution A : [H+] Solution B = 0.001 M:0.00001 M
To simplify this ratio, we can express both concentrations in the same units (e.g., both in millimoles per litre, mM):
[H+] Solution A : [H+] Solution B = 1 mM:0.01 mM
Solution A has a higher hydrogen ion concentration (10 times higher) than Solution B, indicating that it is more acidic. The pH scale is inversely proportional to [H+], meaning lower pH values correspond to higher [H+] concentrations and stronger acidity.
2. Classify the following substances as acid or base:
I. Mg(OH)2
II. HCl
III. KOH
IV. H3PO4
V. HNO3
a) Acid: II, IV, V; Base: I, III
b) Acid: I, II, V; Base: III, IV
c) Acid: I, III, V; Base: II, IV
d) Acid: I, III; Base: II, IV, V
Answer: a) Acid: II, IV, V; Base: I, III
3. When nitric acid (HNO3) reacts with magnesium (Mg), what are the products of the reaction, and what type of reaction is it?
a) Products: Magnesium nitrate (Mg(NO3)2) and hydrogen gas (H2); Single-displacement reaction
b) Products: Magnesium oxide (MgO) and nitrogen gas (N2); Synthesis reaction
c) Products: Magnesium chloride (MgCl2) and water (H2O); Double-displacement reaction
d) Products: Magnesium sulphate (MgSO4) and oxygen gas (O2); Decomposition reaction
Answer: a) In the reaction between nitric acid (HNO3) and magnesium (Mg), a single-displacement reaction occurs, where the magnesium displaces the hydrogen from the nitric acid, forming magnesium nitrate and hydrogen gas as products.
4. A student mixes a strong acid (pH 1) with a strong base (pH 13) in a beaker. The resulting pH is 7. Explain why this pH is neutral, even though the original solutions were strongly acidic and basic.
a) The pH meter is malfunctioning.
b) The solutions have completely neutralised each other.
c) The student made an error in measuring the pH.
d) This result is not possible; it must be an experimental error.
Answer: c) When a strong acid and a strong base are mixed in equal amounts, they undergo a neutralisation reaction, resulting in a pH of 7, which is considered neutral. The hydrogen ions (H+) from the acid and the hydroxide ions (OH-) from the base combine to form water (H2O), effectively neutralising the acidity and basicity of the original solutions.
5. In the dissociation of hydrochloric acid (HCl) in water, what role does water play in the formation of hydronium ions?
a) Water acts as a catalyst in the dissociation process.
b) Water provides electrons to form hydronium ions.
c) Water attracts chloride ions to form hydronium ions.
d) Water attracts hydrogen ions to form hydronium ions.
Answer: d) In the dissociation of hydrochloric acid (HCl) in water, water molecules play a crucial role in attracting and stabilising the positively charged hydrogen ions (H+), forming hydronium ions (H3O+). Water's partial negative charge on its oxygen atom attracts the positively charged H+ ion, leading to the formation of H3O+ ions. This process is essential in aqueous acid solutions and represents the actual behaviour of hydrogen ions in water.
FAQs
1. What is an acid and how is it defined?
In a chemical process, an acid is a material that may provide or receive a proton and an electron pair. To put it another way, when acids dissolve in water, they produce hydrogen ions (H+).
2. What are some examples of common acids found in everyday life?
Common acids include the hydrochloric acid found in the stomach, the acetic acid found in vinegar, and the citric acid found in citrus fruits.
3. What are the properties of acids?
Acids generally taste sour, react with metals to release hydrogen gas, and turn blue litmus paper red. When dissolved in water, they transmit electricity as well.
4. What separates an acid from a base?
Substances that may contribute electron pairs or accept protons are known as bases. They have the tendency to taste bitter, feel slick, turn red litmus paper blue, and are well-known for their acid-neutralizing properties.
5. How do acids and bases react to form salts?
A base and an acid react to neutralise one another, creating salt and water in the process. The process is called this process neutralisation.
