3.1.2 - Amount of Substance

Question 1

Define the mole.

Answer 1

The mole is the amount of substance that contains as many particles (atoms, molecules, or ions) as there are atoms in 12 grams of carbon-12.

Question 2

What is Avogadro’s constant?

Answer 2

Avogadro’s constant is 6.022 x 10^23, representing the number of particles in one mole of any substance.

Question 3

How is the molar mass of a compound calculated?

Answer 3

Molar mass is calculated by adding up the atomic masses (from the periodic table) of each element in the compound.

Question 4

Calculate the number of moles in 35.0g of CuSO4.

Answer 4

Moles = mass / Mr = 35.0 / (63.5 + 32.0 + 16.0 x 4) = 0.219 mol.

Question 5

Define relative atomic mass (Ar).

Answer 5

Relative atomic mass is the average mass of one atom of an element compared to one twelfth of the mass of one atom of carbon-12.

Question 6

Define relative molecular mass (Mr).

Answer 6

Relative molecular mass is the average mass of a molecule compared to one twelfth of the mass of one atom of carbon-12.

Question 7

Calculate the number of chloride ions in 25.0 cm^3 of a 0.400 mol/dm^3 MgCl2 solution.

Answer 7

Moles of MgCl2 = 0.400 x 0.025 = 0.0100 mol. Moles of Cl- = 0.0100 x 2 = 0.0200 mol. Number of Cl- ions = 0.0200 x 6.022 x 10^23 = 1.20 x 10^22 ions.

Question 8

What is the role of density in calculations involving pure liquids?

Answer 8

Density is used to calculate the mass from a known volume of a liquid, essential for determining moles and other properties.

Question 9

Calculate the density of gold if 980 mol weigh 193.06 kg and occupy a volume of 10 dm^3.

Answer 9

Density = mass / volume = 193.06 kg / 10 dm^3 = 19.3 kg/dm^3.

Question 10

What steps are involved in converting the empirical formula to the molecular formula?

Answer 10

Calculate how many times the mass of the empirical formula fits into the Mr. Multiply the subscripts of the empirical formula by this factor to get the molecular formula.

Question 11

How do you calculate the moles of a gas using the ideal gas equation PV = nRT?

Answer 11

Moles (n) = PV / RT. Ensure all units are correct: P in Pa, V in m^3, R as 8.31 J/K·mol, and T in K.

Question 12

Explain how to determine the number of molecules in a given volume of ethanol with a density of 0.789 g/cm^3.

Answer 12

First calculate mass using density x volume. Then, calculate moles using mass / Mr. Finally, multiply moles by Avogadro’s constant to find the number of molecules.

Question 13

What are the steps to calculate the concentration of a solution from mass and volume?

Answer 13

First, calculate moles from mass divided by molar mass (Mr). Then, calculate concentration by dividing moles by volume in dm^3.

Question 14

How is the concentration of ions in a solution determined when the solute dissociates into multiple ions?

Answer 14

Calculate the moles of the solute, then use the stoichiometry of the dissociation to find moles of each ion. Divide moles of ions by the volume of solution to find their concentrations.

Question 15

Explain the procedure for making a standard solution in a volumetric flask.

Answer 15

Dissolve the solute in a beaker with a portion of solvent, transfer to a volumetric flask using a funnel, rinse the beaker and funnel, add solvent up to the calibration mark on the flask, and mix thoroughly.

Question 16

How do you perform a dilution calculation for a solution?

Answer 16

Use the formula C1V1 = C2V2, where C1 and V1 are the initial concentration and volume, and C2 and V2 are the concentration and volume after dilution.

Question 17

Describe the proper technique for conducting a titration.

Answer 17

Rinse and fill the burette with the titrant, add the analyte to a conical flask, add a few drops of suitable indicator, titrate until the end point is reached, and record the volume of titrant used.

Question 18

What is the formula to convert °C to K?

Answer 18

To convert from Celsius to Kelvin, add 273 to the Celsius temperature.

Question 19

Calculate the mass of ethanol that can be obtained from 0.500 dm^3 of ethanol with a density of 0.789 g/cm^3.

Answer 19

Mass = density x volume = 0.789 g/cm^3 x 500 cm^3 = 394.5 g.

Question 20

What are the unit conversions for volume in the context of chemistry calculations?

Answer 20

1 cm^3 = 0.001 dm^3, 1 dm^3 = 0.001 m^3, 1 cm^3 = 1 mL, 1 dm^3 = 1 liter.

Question 21

Explain how to calculate the number of molecules in a given mass of a substance.

Answer 21

First, calculate moles from mass divided by molar mass (Mr). Then multiply moles by Avogadro’s constant (6.022 x 10^23) to find the number of molecules.

Question 22

What is the procedure to calculate the water of crystallization in a hydrated salt?

Answer 22

Weigh the hydrated salt, heat to remove water, weigh again to get the mass of the anhydrous salt. The difference in mass gives the mass of water lost. Calculate moles of water and salt to find the ratio.

Question 23

Calculate the number of moles of H2O in hydrated zinc sulfate if 3.51 g yields 1.97 g of anhydrous ZnSO4.

Answer 23

Moles of H2O = (mass of H2O / Molar mass of H2O) = (1.54 g / 18 g/mol) = 0.085 mol.

Question 24

How do you calculate the density of a substance?

Answer 24

Density = mass / volume.

Question 25

How is molar concentration calculated?

Answer 25

Molar concentration (mol/dm^3) = moles of solute / volume of solution in dm^3.

Question 26

Describe how to prepare a standard solution for use in titrations.

Answer 26

Weigh the precise amount of solute, dissolve in a beaker with solvent, transfer to a volumetric flask using a funnel, rinse all equipment into the flask, fill to the calibration mark, and mix.

Question 27

What are the steps for performing a dilution of a solution?

Answer 27

Measure a specific volume of the original solution, transfer it to a new container, add solvent up to the desired final volume, and mix thoroughly to ensure uniformity.

Question 28

An unknown metal carbonate reacts with hydrochloric acid according to the following equation. M2CO3(aq) + 2 HCl(aq) → 2 MCl(aq) + CO2(g) + H2O(l) A 3.96 g sample of M2CO3 was dissolved in distilled water to make 250 cm3 of solution. A 25.0 cm3 portion of this solution required 32.8 cm3 of 0.175 mol dm–3 hydrochloric acid for complete reaction. Calculate the Mr of M2CO3 and identify the metal M.

Answer 28

1. Calculate the number of moles of HCl used. 2 mol HCl : 1 mol M2CO3 So 0.00574 NaOH : 0.00287 mol M2CO3. 3. Calculate the number of moles M2CO3 acid in original 250 cm3 of solution. Moles in 250 cm3 = 0.00287 x 10 = 0.0287 mol. 4. Work out the Mr of M2CO3 Mr= mass / moles = 3.96/ 0.0287 = 138.0. 5. Work out Ar of M = (138-12- 16x3) / 2 Ar of M = 39 M= potassium.

Question 29

What is the formula for hydrogen peroxide?

Answer 29

H2O2

Question 30

In a titration, it is important to wash the inside of the titration flask with distilled or deionised water as you approach the end-point. Suggest one reason why it’s important to wash the inside of the flask.

Answer 30

To make sure all of the solutions react with each other.

Question 31

Suggest why washing with water does not affect the titre value.

Answer 31

Does not change the moles of the reactants.

Question 32

What experimental data are required in order to calculate the empirical formula of a compound?

Answer 32

Mass of each element in the compound.

Question 33

What are concordant results?

Answer 33

Results within 0.1 cm³ of each other.