Acid Base and Redox Equilibria Questions and Answers

Answer: D

The pH is 12.

KOH is a strong base and dissociates completely, so \([OH^{-}] = 10^{-2}\) M.

\[ \begin{aligned} pOH &= -\log(10^{-2}) = 2 \\ pH &= 14 - pOH = 14 - 2 = 12 \end{aligned} \]

2 is the pOH, or the pH of \(10^{-2}\) M acid, which comes from forgetting that KOH is a base. \(-2\) and \(-12\) come from dropping the minus sign in the logarithm; a dilute solution like this cannot have a negative pH.

Answer: A

The completely ionizing acids are HCl and HNO\(_{3}\).

Hydrochloric acid and nitric acid are strong acids: in dilute aqueous solution practically every molecule gives up its proton, so they ionize completely.

Carbonic acid (H₂CO₃) and acetic acid (CH₃COOH) are weak acids that ionize only slightly; a 0.1 M acetic acid solution is about 1 per cent ionized. Every other option pairs at least one of these weak acids, so only option A contains two acids that ionize completely.

Answer: B

Four units of \(\mathrm{CH_{2}O}\) give \(\mathrm{C_{4}H_{8}O_{4}}\).

Multiplying the empirical formula by 4 multiplies each subscript by 4:

\[ (\mathrm{CH_{2}O})_{4} = \mathrm{C_{4}H_{8}O_{4}} \]

The ratio C : H : O stays 1 : 2 : 1. A tetrose sugar such as erythrose has this formula.

\(\mathrm{C_{4}H_{16}O_{4}}\) has twice the hydrogen of four \(\mathrm{CH_{2}O}\) units, and \(\mathrm{C_{4}H_{6}O_{6}}\) and \(\mathrm{C_{4}H_{10}O_{8}}\) do not have the 1 : 2 : 1 ratio at all.

Answer: C

122 g of iron is 2.18 mol Fe.

The number of moles is mass divided by molar mass (the paper prints '122 of Fe', meaning 122 g):

\[ n = \frac{122}{55.858} \approx 2.18\ \text{mol} \]

As a check, 2 mol of iron is about 111.7 g and 2.2 mol about 122.9 g, so 122 g lies just below 2.2 mol.

1.28 and 1.18 mol would weigh only about 66 to 72 g, and 8.12 mol would be about 454 g.

Answer: B

\(\mathrm{Fe^{++}}\) has 24 electrons.

Iron has atomic number 26, so a neutral iron atom has 26 electrons. The ferrous ion \(\mathrm{Fe^{2+}}\) is formed by losing two electrons (the two 4s electrons), leaving

\[ 26 - 2 = 24 \]

with configuration [Ar] 3d6.

26 is the neutral atom, 23 would be the ferric ion \(\mathrm{Fe^{3+}}\), and 18 and 16 do not correspond to any common ion of iron.

Answer: B

Molar mass is the mass in 1 mol of a substance.

It is expressed in grams per mole and is numerically equal to the relative molecular mass: water has molar mass 18 g/mol, so one mole of water, 6.022 × 10²³ molecules, weighs 18 g.

Mass per litre of solvent is a concentration (and grams of solute per kilogram of solvent leads to molality), not a property of the compound itself. Mass in 1 ml of solvent is closer to density, and 'mass in 1 g of a compound' is circular, since it is simply 1 g.

Answer: A

The 63 g contains \(6.022 \times 10^{23}\) HNO₃ molecules.

The molar mass of nitric acid is

\[ 1 + 14 + 3 \times 16 = 63\ \text{g/mol} \]

so 63 g of HNO₃ is exactly one mole, and one mole of any substance contains Avogadro's number of particles, \(6.022 \times 10^{23}\).

A single molecule weighs only about \(1.05 \times 10^{-22}\) g, so 1, 100 or 1000 molecules are vanishingly small masses, nowhere near 63 g.

Answer: B

The metal that does not produce hydrogen even at red heat is Be.

Beryllium has a very small atom and a high ionisation energy, and it is protected by a thin, tough film of beryllium oxide. As a result it does not react with water or with steam even when red hot, an exception in Group 2.

The others all give hydrogen. Lithium reacts steadily with cold water, forming lithium hydroxide. Calcium reacts readily with cold water. Magnesium reacts very slowly with cold water but readily with hot water and burns in steam to give magnesium oxide and hydrogen.

Answer: B

One molecule of pentane has 12 hydrogen atoms.

Pentane is an alkane, and alkanes follow the general formula C_nH_2n+2. With five carbon atoms:

\[ 2n + 2 = 2(5) + 2 = 12 \]

so the formula is C₅H₁₂, CH₃CH₂CH₂CH₂CH₃ for the straight chain.

5 is the number of carbon atoms, not hydrogens. 17 is the total number of atoms in the molecule (5 + 12), and 9 does not follow from any alkane formula.

Answer: B

Ba⁺⁺ has 54 electrons.

Barium has atomic number 56, so a neutral atom has 56 electrons. The ion Ba²⁺ has lost its two outer 6s electrons:

\[ 56 - 2 = 54 \]

which gives it the stable electron configuration of the noble gas xenon.

56 is the count for the neutral atom. 46 would mean losing ten electrons, and 62 would require gaining six, the opposite of what a positive charge means.