Transcript Topic 8 - whitburnscience

Topic 8
Acids and Alkalis
Starter
1. Name the oxide substance that can be
used to form an acid.
2. a) What is produced at the negative
electrode when an acid is electrolysed?
b) How can you test for this?
3. What ions are present in alkalis?
4. What happens to the pH and concentration
of hydrogen ions when an acid is diluted?
5. From which groups can soluble metal
hydroxides be formed?
Calculations
Gram Formula Mass
• This is the total mass of a compound.
• It is the mass of one mole of a substance.
• Sometimes called Molar Mass.
• It takes into consideration the number and
mass of each element in a compound.
Steps to Calculate Formula Mass
1. Correct Formula – Use valency rules!
2. Write down numbers of each elements
in compound.
3. Add in mass of element – use pg4 in
data book.
4. Add up total mass.
Example 1
Q. Work out the formula mass of sodium
chloride
1. Work out Formula – Na Cl
Cross over valencies– NaCl
2. Numbers of each element
(1 x Na) + (1 x Cl)
3. Add in mass
(1 x 23) + (1 x 35.5)
4. Add total mass
23 + 35.5 = 58.5 amu.
Example 2
Q. Work out the formula mass of Calcium
hydroxide.
1. Work out Formula – Ca OH
Cross over valencies– Ca(OH)2
2. Numbers of each element
(1 x Ca) + (2 x O) + (2 x H)
3. Add in mass
(1 x 40) + (2 x 16) + (2 x 1)
4. Add total mass
40 + 32 + 2 = 74 amu.
Example 3
Q. Work out the formula mass of magnesium
phosphate
1. Work out Formula – Mg PO4
Cross over valencies– Mg3(PO4)2
2. Numbers of each element
(3 x Mg) + (2 x P) + (8 x O)
3. Add in mass
(3 x 24.5) + (2 x 31) + (8 x 16)
4. Add total mass
73.5 + 62 + 128 = 263.5 amu.
The Mole
m
n FM
m = mass
n = number of moles
FM = Formula Mass
Knowing any 2 of these factors allows you to
calculate the third unknown:
m = n x FM
n= m
FM
FM = m
n
Example 1
Q. Calculate the mass of 2 Moles of water
1. Work out FM for 1 mole
a) Formula – H2O
b) Numbers of elements – (2 x H) + (1 x O)
c) Add in Mass - (2x1) + (1x 16)
d) Formula Mass – (2x1) + 16 = 18 g
2. Use triangle
m
n
FM
m= n x FM
m = 2 x 18 = 36g
Example 2
Q. Calculate the number of moles of 11.7g
of sodium chloride.
1. Work out FM for 1 mole
a) Formula – NaCl
b) Numbers of elements – (1 x Na) + (1 xCl)
c) Add in Mass - (1x23) + (1x35.5)
d) Formula Mass – (23) + 35.5 = 58.5 g
2. Use triangle
n
m
FM
n= m = 11.7 = 0.2 moles
FM 58.5
Example 3
Q. Calculate the number of moles 2.6g of
calcium fluoride .
1. Work out FM for 1 mole
a) Formula – CaF2
b) Numbers of elements = (1 xCa) + (2 xF)
c) Add in Mass = (1x40) + (2x19)
d) Formula Mass = 40 + 38 = 78 g
2. Use triangle
n
m n= m = 2.6= 0.033 moles
FM
FM 78