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iGCSE Chemistry: Calculations - core

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Explain why atoms are neutral
The numbers of electrons and protons are equal
State the mass and charge of a neutron
Neutron: mass=1, charge =0
State the mass and charge of a proton
Proton: mass=1, charge=+1
State the mass and charge of an electron
Electron: mass=almost zero, charge=-1
State the particle that is found in the shells orbiting the nucleus
Electrons
State the particles that are found within the nucleus of an atom
Protons and neutrons
How many neutrons in a typical atom of fluorine?
10
How many protons in a fluorine atom?
9
Tritium, an isotope of hydrogen, has a mass number of 3. How many neutrons does an atom of tritium contain?
2
What are isotopes?
Isotopes are atoms of the same element (same number of protons) but with a different number of neutrons (different mass)
What is a mass number?
Mass number is the number of protons plus the number of neutrons in an atom
What is an atomic number?
The atomic number is the number of protons in an atom
What is meant by the term relative atomic mass, Aᵣ?
The average mass of an atom of an element, relative to 1/12th of the mass of an atom of carbon-12
What is the atomic number of a chlorine atom?
17
What is the relative atomic mass (Aᵣ) of a potassium atom?
39
A sample of bromine contained the two isotopes in the following proportions: bromine-79 = 50.7% and bromine-81 = 49.3%. Calculate the relative atomic mass of bromine
((79x50.7)+(81x49.3))/100 = 79.99
A sample of carbon contained 98.90% carbon-12 and 1.10% carbon-13. Calculate the relative atomic mass of carbon
((12x98.90)+(13x1.10))/100 = 12.01
How many electron shells do the elements in the third period have?
3
How many electrons can the first, second and third shells hold?
First 2, second 8, third 8
How many electrons in a calcium atom?
20
How many electrons in the outer shell of group 7 elements?
7
Write the electronic configuration of potassium
2,8,8,1
Calculate the relative formula mass(Mr) of ammonium nitrate (NH₄NO₃)
80
Calculate the relative formula mass(Mr) of calcium hydroxide (Ca(OH)₂)
74
Calculate the relative formula mass(Mr) of hydrated copper (II) sulfate (CuSO₄.5H₂O)
249.5
State the steps for calculating empirical formula
1) mass/Ar. 2) divide by the smallest. 3) Give the ratio has a whole number. 4) State final empirial formula
Tungsten has the chemical symbol W. Suggest the chemical name of WO₃.
tungsten oxide
What is the empirical formula of an alcohol with the analysis; 52.2% C, 13.0% H and 34.8% O?
C₂H₆O
What is the empirical formula of a compound in which 0.48 g of carbon combines with 0.08 g of hydrogen and 0.64 g of oxygen?
CH₂O
Work out the empirical formula of an oxide of chlorine contains 7.1 g of chlorine and 1.6 g of oxygen.
Cl₂O
What is meant by the term empirical formula?
A chemical formula that shows the simplest ratio of the numbers of atoms in a compound
What is meant by the term molecular formula?
A chemical formula that shows the actual numbers of the different types of atoms in a molecule
A compound that contained 24.24% Carbon. 4.04% Hydrogen and 71.72% Chlorine and has a relative molecular mass of 99. Calculate the empirical formula and the molecular formula.
Empirical formula is CH₂Cl. Molecular formula is also C₂H₄Cl₂
An oxide of nitrogen contains 26% nitrogen and 74% oxygen and has a relative molecular mass of 108. Find the empirical and molecular formulae for the oxide.
Empirical formula is N₂O₅. Molecular formula is also N₂O₅
To determine the formula of a metal oxide by combustion, magnesium is heated in a crucible. Why is a lid lifted from time to time?
To allow oxygen in so the magnesium fully reacts
To determine the formula of a metal oxide by combustion, magnesium is heated in a crucible. Why is a lid used?
To stop the escape of magnesium oxide smoke
To determine the formula of a metal oxide by combustion, magnesium is heated in a crucible. Why is the crucible is cooled, weighed, reheated, cooled, re-weighted, and so on?
By repeating till 2 consecutive masses are recorded, we know that the magnesium has all reacted