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Hazardous Materials Otsu-4 | Calculations with Moles, Combustion Equations, Neutralisation, and Gas Laws

1 / 100.0s

Problem 1

A sample contains 9.2 g of pure ethanol, C₂H₆O. Use atomic masses C = 12, H = 1.0, and O = 16, and assume no sample loss.

What is the amount of ethanol in the sample?

View explanation

Ethanol's molar mass is 12 × 2 + 1.0 × 6 + 16 = 46 g/mol. Amount of substance equals mass divided by molar mass, so 9.2 ÷ 46 = 0.20 mol. A result of 0.10 mol can arise from double-counting part of the formula, while 46 is the numerical value of the molar mass, not the amount in moles.

Problem 2

Ethanol, C₂H₅OH, burns completely in excess oxygen to form only carbon dioxide and water. Express the equation using the smallest whole-number coefficients.

In C₂H₅OH + aO₂ → bCO₂ + cH₂O, which values of a, b, and c are correct?

View explanation

Two carbon atoms require 2CO₂, and six hydrogen atoms require 3H₂O. The products then contain seven oxygen atoms; ethanol supplies one, leaving six to be supplied by 3O₂. The balanced equation is C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. Subscripts in molecular formulas must not be changed to balance an equation.

Problem 3

A mixture of 2.0 mol methane and 3.0 mol oxygen reacts completely according to CH₄ + 2O₂ → CO₂ + 2H₂O, with no side reactions.

Which combination correctly identifies the limiting reactant and the amount of the other reactant left after the reaction?

View explanation

One mole of methane requires two moles of oxygen. Thus 3.0 mol oxygen can react with 3.0 ÷ 2 = 1.5 mol methane, making oxygen the limiting reactant. Of the initial 2.0 mol methane, 1.5 mol reacts and 0.50 mol remains. The reactants must be compared through the 1:2 equation ratio, not simply by their initial mole quantities.

Problem 4

Propane burns completely according to C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Twelve litres of propane are burned at the same temperature and pressure as the supplied gases. Treat oxygen as 20% of air by volume and assume ideal complete reaction.

What volume of air is required for complete combustion?

View explanation

At equal temperature and pressure, gas-volume ratios follow the equation coefficients, so the oxygen requirement is 12 L × 5 = 60 L. If oxygen is 20% of air, the required air volume is 60 ÷ 0.20 = 300 L. Sixty litres is the amount of oxygen itself, not the amount of air.

Problem 5

A 500 mL solution contains 25% by mass of a combustible component. The solution's density is 0.80 g/mL, and none of the component is lost.

What mass of the combustible component does the solution contain?

View explanation

First find the total solution mass: 0.80 g/mL × 500 mL = 400 g. The component is 25% of that mass, so 400 × 0.25 = 100 g. Multiplying 500 directly by 25% gives 125, but that incorrectly treats a volume in millilitres as a mass in grams and ignores density. Four hundred grams is the solution mass, not the solute mass.

Problem 6

A 100 mL portion of 0.50 mol/L sulfuric acid, H₂SO₄, is completely neutralised with 1.0 mol/L sodium hydroxide, NaOH. Assume H₂SO₄ supplies two hydrogen ions per formula unit and NaOH supplies one hydroxide ion.

What volume of the NaOH solution is required?

View explanation

The sulfuric acid amount is 0.50 mol/L × 0.100 L = 0.050 mol. Each mole of sulfuric acid requires two moles of NaOH, so 0.100 mol NaOH is needed. At 1.0 mol/L, that amount occupies 0.100 L, or 100 mL. Treating every acid-base reaction as a 1:1 mole ratio would incorrectly give 50 mL.

Problem 7

Consider the reduction of copper(II) oxide by carbon monoxide: CO + CuO → CO₂ + Cu. Carbon changes from oxidation state +2 to +4, while copper changes from +2 to 0.

Which statement correctly describes the oxidation, reduction, and roles of the substances?

View explanation

Carbon's oxidation state increases from +2 to +4, so CO is oxidised. Because it causes CuO to be reduced, CO acts as the reducing agent. Copper in CuO decreases from +2 to 0 and is reduced, meaning CuO acts as the oxidising agent. A common source of confusion is that the oxidising agent is itself reduced.

Problem 8

A 4.6 g sample of pure ethanol burns completely. Use a molar mass of 46 g/mol and a combustion enthalpy of −1,360 kJ per mole, and assume no heat loss.

Which statement correctly gives the heat transfer and its safety significance?

View explanation

The 4.6 g sample is 4.6 ÷ 46 = 0.10 mol. The magnitude of heat released is 1,360 × 0.10 = 136 kJ; the negative reaction enthalpy denotes an exothermic reaction that transfers heat from the system to the surroundings. Thermal effects on nearby materials and the container must therefore be considered.

Problem 9

Gas in a rigid sealed vessel is at 1.2 MPa and 27°C. Its amount remains constant, the gas behaves ideally, and it is heated uniformly to 127°C.

Which combination correctly states the final pressure and the resulting safety consideration?

View explanation

At constant volume and amount, pressure is proportional to absolute temperature. Since 27°C = 300 K and 127°C = 400 K, P₂ = 1.2 × 400 ÷ 300 = 1.6 MPa. The ratio 127 ÷ 27 cannot be used because Celsius temperature is not an absolute scale. Heating a closed vessel can raise gas pressure as well as expand its liquid contents.

Problem 10

A 0.40 kg quantity of liquid evaporates completely while remaining at constant temperature. Its latent heat of vaporisation is 250 kJ/kg; ignore all heat transfer other than evaporation.

Which statement correctly gives the heat received by the liquid and explains the phase change?

View explanation

The heat absorbed during evaporation is Q = mL = 0.40 kg × 250 kJ/kg = 100 kJ. This energy is used to overcome intermolecular attraction as the liquid becomes gas, so heat transfer occurs even though the temperature remains constant during the phase change. Condensation transfers the same latent heat in the opposite direction, to the surroundings.