unit 1 Chemical calculations topic 5&6
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What information does a chemical’s formula tell you?
It gives the number of atoms present in the molecule. The formula of a covalent network or ionic compound gives the simplest ratio of atoms/ions in the substance.
What do the following prefixes mean? • mono • di • tri • tetra
• one atom • two atoms • three atoms • four atoms
What is the formula for: Sulphur Monoxide Carbon Dichloride Copper Tribromide Phosphorus Tetrahydride
SO CCl2 CuBr3 PH4
What is meant by the valency of a chemical?
Valency is the combining power of an element. Elements in the same group of the periodic table have the same valency. The valency of an element is related to how many electrons are in the outer shell.
What are the valencies of elements in groups 1 to 8?
12343210
If given the name of a compound, how can you work out the formula?
S.V.S.D.F system. (Valency cross over) Step one - write down the symbols Step two - write valency (USE DATA BOOKLET) Step three - swap the valencies Step four - if the valencies can be simplified, divide them both by the smaller of the two numbers. If one of the numbers is already one, then they cannot be divided and simplified any further. Step five - write the formula.
What is the formula of strontium chloride?
SrCl2
What is the formula of Germanium sulfide?
GeS2
How can you find out the valency of group ions?
In the data booklet (page 8) We can take the number of charges on the ion as the valency of the ion. OH- has a valency of 1 SO42- has a valency of 2
What is the formula of Magnesium Carbonate?
MgCO3
Work out the formula of Beryllium Phosphate.
Be3(PO4)2
Why do some compounds have roman numerals in their name? Copper (II) chloride
Some elements do not always make the same number of bonds. These include some transition metals. The valency of these elements is usually given in roman numerals inside brackets.
What is the formula of: • Copper (II) chloride? • Copper (I) chloride?
• CuCl2 • CuCl
What must be included when asked to write an ionic formula?
Charges on each ion. One part will be positive and the other negative.
What is the ionic formula for potassium nitrate?
Potassium makes 1 bond and will be positive. Nitrate makes 1 bond and has a negative charge (from data booklet) Formula = KNO3 Ionic formula = K+NO3-
What is the ionic formula for iron(III) oxide?
Iron makes 3 bonds and will be positive Oxygen makes 2 bonds and will be negative Formula = Fe2O3 Ionic formula = (Fe3+)2(O2-)3
What is meant by the gram formula mass (GFM) of a substance?
The GFM is the mass of one mole of a substance
How do you calculate the Gram Formula Mass (GFM) of a compound?
Work out the formula then add all the mass numbers together.
What is the GFM of Carbon Dioxide?
CO2 12 + (2x16) = 44
To calculate the mass of a number of moles, or calculate the number of moles in a certain mass, a formula triangle can be used. Describe it.
n=m/gfm
How many moles are present in 25 g of calcium carbonate? (GFM=100)
number of moles = mass/GFM 25/100 = 0.25 moles
Calculate the mass of 0.25 moles of butane (C4H10) RAM of Carbon = 12 RAM of Hydrogen = 1
mass = number of moles x GFM mass = 0.25 x 58 mass = 14.5g
Equations can be described as being balanced. What does this mean?
A chemical equation is said to be balanced when there are the same number of the same type of every atom on both sides of the equation.
How can you balance an equation?
Balancing is adding BIG numbers. You cannot change any of the small numbers in a chemical formula. If balancing is required, put the number in front of the substance.