Unit 4 Practice: Hard

Answer: 4 A₂B particles form, with 1 B particle left over.

Explanation: Every product requires:

2 A + 1 B

8 A can make:

8 ÷ 2 = 4 products

That uses 4 B.

Since 5 B were available, 1 B remains.

A is the limiting reactant.

Answer: Coefficients change the quantity of each substance, while subscripts change the identity of the substance.

Explanation: Changing:

H₂O → 2H₂O

means two water molecules.

Changing:

H₂O → H₂O₂

creates hydrogen peroxide, a completely different compound.

Answer: About 80.0%

Work:

Moles CO₂:

3.52 g / 44.01 g/mol ≈ 0.0800 mol

Mole ratio:

1 mol CaCO₃ : 1 mol CO₂

So:

0.0800 mol CaCO₃

Mass CaCO₃:

0.0800 × 100.09 ≈ 8.01 g

Percent:

(8.01 / 10.0)(100) ≈ 80.1%

Answer: O₂ is limiting; about 22.5 g H₂O form.

Work:

H₂:

5.00/2.016 ≈ 2.48 mol

O₂:

20.0/32.00 = 0.625 mol

0.625 mol O₂ requires:

1.25 mol H₂

More H₂ than that is available, so O₂ is limiting.

Water:

0.625 mol O₂ × 2 = 1.25 mol H₂O

Mass:

1.25 × 18.02 ≈ 22.5 g

Answer: Use PV=nRT to find moles of gas, then use the balanced equation's mole ratio.

Explanation: Gas measurements themselves do not directly give stoichiometric amounts.

First:

n = PV/RT

Then use the mole ratio to connect the gas to another reactant or product.

Answer: The balanced equation may require different amounts of each reactant.

Explanation: For:

N₂ + 3H₂ → 2NH₃

3 mol H₂ are required for every 1 mol N₂.

So a reactant can have more moles but still be limiting if the reaction requires it in a larger proportion.

Answer:

H = +1

O = −2

S = +4

Work:

Neutral compound:

2(+1) + S + 3(−2) = 0

2 + S − 6 = 0

S = +4

Answer:

Fe²⁺ is oxidized.

Cl₂ is reduced.

Explanation: Fe:

+2 → +3

It loses electrons.

Cl:

0 → −1

It gains electrons.

Answer:

Oxidizing agent: Cl₂

Reducing agent: Fe²⁺

Explanation: Cl₂ gets reduced while causing Fe²⁺ to oxidize.

Fe²⁺ gets oxidized while causing Cl₂ to reduce.

Answer:

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Explanation: Balance O using H₂O:

4 O → add 4H₂O to products.

Balance H:

4H₂O contains 8H → add 8H⁺ to reactants.

Now charges:

Left before electrons:

−1 + 8 = +7

Right:

+2

Add 5e⁻ to the left:

+7 − 5 = +2

Charge is balanced.

Answer: Add OH⁻ to both sides to neutralize every H⁺, form H₂O, and then cancel excess water.

Explanation: For every H⁺ present, add the same amount of OH⁻ to both sides.

Then:

H⁺ + OH⁻ → H₂O

Afterward simplify by canceling water if it appears on both sides.

Answer: 3 electrons

Explanation: The oxidation number decreases by 3.

Reduction means electron gain.

Therefore the atom gains 3 electrons.

Answer: Because it accepts electrons from another species.

Explanation: The oxidizing agent causes another species to lose electrons.

Those electrons have to go somewhere.

The oxidizing agent gains them and is therefore reduced.

Answer: 0.0250 mol H₂SO₄

Work:

Ratio:

1 mol H₂SO₄ : 2 mol NaOH

So:

0.0500 × 1/2 = 0.0250 mol

Answer: 0.120 M

Work:

Moles NaOH:

0.150 × 0.0200 = 0.00300 mol

1:1 ratio:

0.00300 mol acid

Volume acid:

0.0250 L

Molarity:

0.00300 / 0.0250 = 0.120 M

Answer: 0.0600 M

Explanation: If:

1 mol acid : 2 mol NaOH

then acid moles:

0.00300 / 2 = 0.00150 mol

Then:

0.00150 / 0.0250 = 0.0600 M

Answer:

Equivalence point: exact stoichiometric reaction point.

Endpoint: observed experimental signal telling you to stop the titration.

Explanation: An endpoint may be an indicator color change.

A good indicator is chosen so its endpoint occurs very close to the true equivalence point.

Answer: The products may react with water and make the solution acidic or basic.

Explanation: For strong acid + strong base at 25°C, equivalence is approximately pH 7.

But a weak acid titrated with strong base forms its conjugate base at equivalence.

That conjugate base can react with water and produce OH⁻, making the solution basic.

Answer: The identities of Na⁺ and Cl⁻ remain unchanged.

Explanation: The ionic lattice is disrupted and the ions become hydrated by water.

But no new chemical identities are created.

Evaporating the water can recover NaCl.

Answer: Gas can also form from a physical process such as boiling or evaporation.

Explanation: Evidence must be interpreted in context.

Gas formation may suggest a chemical reaction, but it is not absolute proof by itself.

A student should determine whether a new substance actually formed.