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REACTING MASSES, AND CHEMICAL EQUATIONS
You can easily name the compound, just by following these rules:
- The name of metal is given first.
- and then the non-metal, but
ending with "ide".
- If it contains a hydrogen then that
is named first, otherwise the one
with the lower group is named first.
- and then the non-metal, but
ending with "ide"
The formula of a compound is related to its structure. For example:
The valency of an element is the number of electrons its atoms lose, gain or share, to form a compound.
This is how to write the formula of a compound, using valencies:
1. Write down their symbols, in the same order as the elements in the name.
2. Write down the valencies of the two elements.
3. Add numbers after the symbols if you need to, to balance the valencies.
EXAMPLE
What is the formula of aluminum oxide?
1. AlO
2. Valencies: aluminum is 3; oxygen is 2.
3. Al O
= Al2O3
3 2
1. When hydrogen reacts with oxygen it gives water. The formula is written like this:
H2 + 02 ----> H20
Now to balance the equation you need to see the number of hydrogen and oxygen atoms on each side.
H2 + 02 ----> H20
= H2 + 02 ----> 2H20
= 2H2 + 02 ----> 2H20
You can show the states of the reactants and products by adding state symbols to the equations:
(s) for solid (l) for liquid
(g) for gas (aq) for aqueous solution
For the reactions below, the equation with state symbols are:
2H2 (g) + O2 (g) ------> 2H2O (l)
EXAMPLE
Q. Magnesium burns in oxygen to form magnesium oxide, a white solid. Write an equation for the reaction.
1. magnesium + oxygen -------> magnesium oxide
2. Mg + O2 -------> MgO
3. The equation is not balanced.
Mg + O2 -------> 2MgO
The equation is still not balanced.
2Mg + O2 -------> 2MgO
The equation is balanced.
4. 2Mg(s) + O2(g) -------> 2MgO(s)
Scientist needed to compare the masses of atoms, so they chose an atom of carbon-12 to be the standard atom. They fixed its mass as exactly 12 atomic mass units.
Then they compared all the other atoms with this standard atom, and found values for their masses.
The atoms of an element are not always identical. Some may have extra neutrons. Different atoms of the same element are
called isotopes.
We need to take all the natural isotopes of an element into account, to
work out the relative atomic mass.
The relative atomic mass (Ar) of an element is the average mass of its isotopes compared to an atom of carbon-12.
Finding the mass of molecules:
If the substance is made of molecules, its mass found in this way is called
the relative molecular mass, or Mr.
Finding the mass of a Ion:
An ion has the same mass as the atom from which it is made.
If the substance is made of ions, its mass is called the relative formula mass, which is also Mr for short.
1. Elements always react in the same ratio, to form a given compound.
EXAMPLE
when carbon burns in oxygen to form carbon dioxide:
6 g of carbon combines with 16 g of oxygen, so
12 g of carbon will combine with 32 g of oxygen.
2. The total mass does not change, during a chemical reaction.
EXAMPLE
12 g of carbon and 32 g of oxygen give 44 g of carbon dioxide.
Calculating quantities is quite easy, using the laws above.
EXAMPLE
Q1. 64 g of copper reacts with 16 g of oxygen to give the black compound copper(II) oxide.
a). What mass of copper will react with 32 g of oxygen?
Ans: - 64 g of copper reacts with 16 g of oxygen, so
C O
64 16 = 128g of copper
? 32
b). What mass of oxygen will react with 32 g of copper?
Ans: - 16 g of oxygen reacts with 64 g of copper, so
C O
64 16 = 8g of oxgyen
32 ?
The percentage composition of a compound tells you how much of each element it contains, as a percentage of the total mass. This is how to work it out:
1. Write down the formula of the compound.
2. Using Ar values, work out its molecular or formula mass (Mr).
3 Write the mass of the element as a fraction of the Mr
4. Multiply the fraction by 100, to give a percentage.
EXAMPLE
Calculate the percentage of oxygen in sulfur dioxide.
1. The formula of sulfur dioxide is SO2.
2. The Mr of the compound is 64 and the Mr of oxygen is 32
3. Mass of oxygen as a fraction of the total = 32/64.
4. Mass of oxygen as a percentage of the total = 32/64 x 100 = 50%
So the compound is 50% oxygen.
A pure substance has nothing else mixed with it.
But substances often contain unwanted substances, or impurities.
Purity is usually given as a percentage. This is how to work it out:
Impure copper is refined (purified), to obtain pure copper for use in computers. 20 tonnes of copper gave 18 tonnes of pure copper, on
refining.
Q. What was the % purity of the copper before refining?
= %purity of the copper = 18 tonnes/20 tonnes x 100 = 90%.
So the copper was 90% pure.