Amounts Of Substances

Question 1

Ions

Answer 1

Silver only forms +1 ions
Zinc only forms 2+ ions
Aluminium only forms 3+ ions

Question 2

Describe how a mass spectrometer works

Answer 2

-In the 1st stage, we take a sample of the element and place it in the sample chamber. This sample contains all of the different isotopes of that element.
-The atoms then go through a process called ionisation where all the atoms are converted into positive ions.
-These + ions are now attracted to a negatively charged plate, causing them to accelerate which increases their KE
(all of the ions with the same charge have the same KE).
-Once the ions pass through the negative plate, they stop accelerating and move towards the detector (the lighter ions reach the detector before heavier ions).
-At the detector, each + ion gains an electron which causes a current to flow and the size of each current produced is used to determine the abundance of each isotope as the current is directly proportional to the abundance.

Question 3

Explain how to interpret a mass spectrum and the interior of the mass spectrometer

Answer 3

The Y-axis shows the percentage abundance.
The X-axis shows the m/z ratio (Ar).
The number of peaks tells us the number of main isotopes.

The interior of the mass spectrometer is a vacuum to prevent the ions from colliding with molecules in the air.

Question 4

Describe what is meant by molecular formula and empirical formula

Answer 4

Molecular formula tells us the elements in a molecule and the number of atoms in each element.

Empirical formula tells us the simplest whole-number ratio of the atoms of each element in a compound.

Question 5

Calculate relative molecular mass and relative formula mass

Answer 5

Relative Molecular Mass is the same as Mr.

Relative Formula Mass is calculated the same way as Mr but we use empirical formula to do so.

Question 6

Define molar mass

Answer 6

Molar mass (M) is the mass of 1 mole of a substance (g mol^-1). For an element, the molar mass has the same value as the relative atomic mass.

Question 7

Use the Avogadro constant to calculate the number of atoms, molecules, ions or electrons in a given sample

Answer 7

To work out the number of atoms, first work out the number of moles and then multiply it by the Avogadro constant.

To work out the number of molecules, we first work out the number of moles and then multiply it by the Avogadro constant. To work out the number of atoms in the molecules we do: num of molecules multiplied by num of atoms present.

Question 8

Carry out calculations based on the concentration of solutions

Answer 8

Concentration = mol / volume (dm^3)

At room temperature and pressure, the molar volume of any gas is 24dm^3

Question 9

Calculate the empirical formula of a compound from experimental data

Answer 9

To work out empirical formula, make the table containing:
-Mass
-Mr
-Moles
-Simplest whole num ratio

Question 10

Work out the molecular formula of a compound from the empirical formula and the relative molecular mass

Answer 10

-First work out the relative molecular mass of the empirical formula.
-Then divide the relative molecular mass that the question gave you by the relative molecular mass of the empirical formula.
-You then take your answer and multiply it by the empirical formula.

Question 11

Describe whats meant by atom economy

Answer 11

Atom economy tells us what percentage of the mass of the reactants that ends up in the product we want.

Question 12

Describe how to work out atom economy

Answer 12

(Mr of desired product / Mr of all reactants) x 100

Question 13

Calculate % yield and explain why it may not be 100%

Answer 13

It may not be 100% if:
-side reactions occur
-some products are lost during filtration
-it is a reversible reaction

To calculate % yield we do:
(Actual yield / theoretical yield) x 100