atoms, molecules and stoichiometry

Question 1

What is the unified atomic mass unit defined as?

Answer 1

One-twelfth of the mass of a carbon-12 isotope

The symbol for the unified atomic mass is u (often Da, Dalton, is used as well) and 1 u = 1.66 x 10^-27 kg.

Question 2

What does the relative atomic mass (Ar) of an element represent?

Answer 2

The ratio of the average mass of the atoms of an element to the unified atomic mass unit.

Question 3

True or False: The relative atomic mass has units.

Answer 3

False

Question 4

Define relative isotopic mass.

Answer 4

The mass of a particular atom of an isotope compared to the unified atomic mass unit.

Question 5

What are isotopes?

Answer 5

Atoms of the same element with a different number of neutrons.

Question 6

How is the average atomic mass of an element calculated?

Answer 6

By considering the percentage abundance of each isotope and calculating a weighted average.

Question 7

What is relative molecular mass (Mr)?

Answer 7

The ratio of the weighted average mass of a molecule of a molecular compound to the unified atomic mass unit.

Question 8

How can the Mr be calculated for a compound?

Answer 8

By adding up the relative atomic masses of all atoms present in one molecule.

Question 9

What is the Avogadro constant (NA or L)?

Answer 9

The number of particles equivalent to the relative atomic mass or molecular mass of a substance, valued at 6.02 x 10^23 g mol^-1.

Question 10

What is one mole of any element equal to in grams?

Answer 10

The relative atomic mass of that element.

Question 11

Fill in the blank: The formula for calculating the relative formula mass (Mr) of ionic compounds is the same as for _______.

Answer 11

[molecular mass]

Question 12

What characterizes ionic compounds?

Answer 12

They are formed from a metal and a nonmetal bonded together.

Question 13

What is the charge of metals in Group 1?

Answer 13

1+

Question 14

What suffix is commonly used for non-metal ions?

Answer 14

‘ide’

Question 15

What is the formula for magnesium chloride?

Answer 15

MgCl2

Question 16

What is a symbol equation?

Answer 16

A shorthand way of describing a chemical reaction using chemical symbols.

Question 17

What must be true for a balanced chemical equation?

Answer 17

The number of each atom on each side of the reaction must be the same.

Question 18

What are spectator ions?

Answer 18

Ions that do not take part in the reaction.

Question 19

What is the empirical formula?

Answer 19

The simplest whole number ratio of the elements present in a molecule or formula unit of the compound.

Question 20

How can the empirical formula be determined from mass?

Answer 20

By calculating the ratio of the masses of each element in the compound.

Question 21

Calculate the moles of hydrogen in a compound that contains 10 g of hydrogen.

Answer 21

10 moles

Question 22

What is the relative atomic mass of oxygen?

Answer 22

16.0

Question 23

What is the empirical formula of a compound that contains 10 g of hydrogen and 80 g of oxygen?

Answer 23

H2O

The empirical formula is derived from the ratio of moles calculated from the masses of each element.

Question 24

How do you calculate moles from mass?

Answer 24

Moles = mass / Ar

Ar stands for atomic mass.

Question 25

What is the empirical formula of a compound that contains 85.7% carbon and 14.3% hydrogen?

Answer 25

CH2 ## Footnote This is calculated by finding the ratio of moles of carbon to hydrogen.

Question 26

What does the molecular formula represent?

Answer 26

The exact number of atoms of each element present in a compound ## Footnote The molecular formula can be derived from the empirical formula.

Question 27

How is the molecular formula calculated from the empirical formula?

Answer 27

Divide the relative formula mass of the compound by the relative formula mass of the empirical formula and multiply each element in the empirical formula by this number ## Footnote This gives the molecular formula.

Question 28

What is water of crystallisation?

Answer 28

Water that is part of the structure of some crystalline compounds ## Footnote Examples include hydrated compounds like CuSO4•5H2O.

Question 29

What is an anhydrous compound?

Answer 29

A compound that does not contain water of crystallisation ## Footnote An example is anhydrous copper(II) sulfate, CuSO4.

Question 30

How can the degree of hydration be calculated?

Answer 30

By measuring the mass of the hydrated salt before and after heating ## Footnote The difference in mass helps determine the number of moles of water.

Question 31

What is the equation for calculating the number of moles?

Answer 31

Number of moles = mass of substance (g) / molar mass (g mol^-1) ## Footnote This equation is crucial for stoichiometric calculations.

Question 32

What is percentage yield?

Answer 32

Percentage yield = (actual yield / predicted yield) x 100 ## Footnote This shows the efficiency of a reaction.

Question 33

What is a limiting reagent?

Answer 33

The reactant that is completely consumed in a chemical reaction, limiting the amount of product formed ## Footnote Identifying the limiting reagent is essential for stoichiometric calculations.

Question 34

What is Avogadro's hypothesis?

Answer 34

Equal volumes of gases contain the same number of molecules ## Footnote This is fundamental for gas calculations.

Question 35

What is the molar gas volume at room temperature and pressure?

Answer 35

24.0 dm3 ## Footnote This is the volume occupied by one mole of any gas at room temperature (20°C) and one atmosphere of pressure.

Question 36

How do you calculate the volume of gas from moles?

Answer 36

Volume of gas (dm3) = amount of gas (mol) x 24.0 ## Footnote This formula allows you to convert moles to volume.

Question 37

What is the concentration of a solution?

Answer 37

The amount of solute dissolved in a solvent to make 1 dm3 of solution ## Footnote The concentration is expressed in mol dm^-3.

Question 38

How do you find the mass of a solute in a solution?

Answer 38

Mass of solute (g) = number of moles (mol) x molar mass (g mol^-1) ## Footnote This requires knowledge of concentration and volume.

Question 39

Calculate the mass of magnesium oxide produced from burning 6 g of magnesium in oxygen.

Answer 39

9.95 g ## Footnote This is determined by using stoichiometry and the molar mass of magnesium oxide.

Question 40

Calculate the percentage yield of copper obtained from 6.54 g of zinc reacting with copper(II) sulfate if 4.80 g of copper was produced.

Answer 40

75.6% ## Footnote This is calculated by comparing the actual yield to the theoretical yield.

Question 41

What is the formula for calculating the volume of hydrochloric acid required to react with calcium carbonate?

Answer 41

Use the balanced equation and concentration to find the volume needed ## Footnote This involves stoichiometric calculations based on the reaction.

Question 42

What is the formula for calculating the mass of solute by its molar mass?

Answer 42

mass of solute (g) = number of moles (mol) x molar mass (g mol-1)

Question 43

What is the balanced symbol equation for the reaction between calcium carbonate and hydrochloric acid?

Answer 43

CaCO3 + 2HCl → CaCl2 + H2O + CO2

Question 44

How do you calculate the number of moles of calcium carbonate from its mass?

Answer 44

Number of moles (CaCO3) = 2.5 g / 100 g mol-1 = 0.025 mol

Question 45

What is the stoichiometric ratio of CaCO3 to HCl in the reaction?

Answer 45

1 mol of CaCO3 requires 2 mol of HCl

Question 46

How many moles of HCl are required for 0.025 mol of CaCO3?

Answer 46

0.05 mol of HCl

Question 47

What is the formula to calculate the volume of HCl required?

Answer 47

Volume (HCl) = amount (mol) / concentration (mol dm-3)

Question 48

What is the volume of hydrochloric acid required to react with 2.5 g of calcium carbonate?

Answer 48

0.05 dm3

Question 49

What is the balanced symbol equation for the reaction of sodium carbonate with hydrochloric acid?

Answer 49

Na2CO3 + 2HCl → Na2Cl2 + H2O + CO2

Question 50

What is the volume of sodium carbonate solution used in the neutralization calculation?

Answer 50

25.0 cm3

Question 51

What is the concentration of the sodium carbonate solution in mol dm-3?

Answer 51

0.050 mol dm-3

Question 52

How do you calculate the number of moles of sodium carbonate reacted?

Answer 52

Number of moles (Na2CO3) = 0.025 dm3 x 0.050 mol dm-3 = 0.00125 mol

Question 53

What is the molar ratio of Na2CO3 to HCl in the reaction?

Answer 53

1 : 2

Question 54

How many moles of HCl react with 0.00125 moles of Na2CO3?

Answer 54

0.00250 moles of HCl

Question 55

What is the formula to calculate the concentration of hydrochloric acid?

Answer 55

Concentration (HCl) = amount (mol) / volume (dm3)

Question 56

What is the concentration of hydrochloric acid calculated in the example?

Answer 56

0.125 mol dm-3

Question 57

What equation relates the number of moles to mass and molar mass?

Answer 57

number of moles = mass of substance (g) / molar mass (g mol-1)

Question 58

What can gas volumes be used to deduce in a reaction?

Answer 58

The stoichiometry of a reaction

Question 59

In the combustion of propane, what is the ratio of propane to oxygen to carbon dioxide?

Answer 59

1 : 5 : 3

Question 60

What is the balanced equation for the combustion of propane?

Answer 60

C3H8 (g) + 5O2 (g) → 3CO2 (g) + H2O (l)