# 6.1 Stoichiometry and composition  (Page 2/5)

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207 g of lead combines with oxygen to form 239 g of a lead oxide. Use this information to work out the formula of the lead oxide (Relative atomic masses: Pb = 207 u and O = 16 u).

1. $239-207=32g$
2. $n=\frac{m}{M}$

$\frac{207}{207}=1mol$

Oxygen

$\frac{32}{16}=2mol$
3. The mole ratio of Pb:O in the product is 1:2, which means that for every atom of lead, there will be two atoms of oxygen. The formula of the compound is PbO ${}_{2}$ .

Vinegar, which is used in our homes, is a dilute form of acetic acid. A sample of acetic acid has the following percentage composition: 39.9% carbon, 6.7% hyrogen and 53.4% oxygen.

1. Determine the empirical formula of acetic acid.
2. Determine the molecular formula of acetic acid if the molar mass of acetic acid is 60g.mol ${}^{-1}$ .
1. In 100g of acetic acid, there is 39.9 g C, 6.7 g H and 53.4 g O

2. $n=\frac{m}{M}$

$\begin{array}{ccc}\hfill {n}_{C}& =& \frac{39.9}{12}=3.33\phantom{\rule{0.166667em}{0ex}}\mathsf{mol}\hfill \\ \hfill {n}_{H}& =& \frac{6.7}{1}=6.7\phantom{\rule{0.166667em}{0ex}}\mathsf{mol}\hfill \\ \hfill {n}_{O}& =& \frac{53.4}{16}=3.34\phantom{\rule{0.166667em}{0ex}}\mathsf{mol}\hfill \end{array}$
3. Empirical formula is CH ${}_{2}$ O

4. The molar mass of acetic acid using the empirical formula is 30 g.mol ${}^{-1}$ . Therefore the actual number of moles of each element must be double what it is in the emprical formula.

The molecular formula is therefore C ${}_{2}$ H ${}_{4}$ O ${}_{2}$ or CH ${}_{3}$ COOH

## Moles and empirical formulae

1. Calcium chloride is produced as the product of a chemical reaction.
1. What is the formula of calcium chloride?
2. What percentage does each of the elements contribute to the mass of a molecule of calcium chloride?
3. If the sample contains 5 g of calcium chloride, what is the mass of calcium in the sample?
4. How many moles of calcium chloride are in the sample?
2. 13g of zinc combines with 6.4g of sulfur.What is the empirical formula of zinc sulfide?
1. What mass of zinc sulfide will be produced?
2. What percentage does each of the elements in zinc sulfide contribute to its mass?
3. Determine the formula of zinc sulfide.
3. A calcium mineral consisted of 29.4% calcium, 23.5% sulphur and 47.1% oxygen by mass. Calculate the empirical formula of the mineral.
4. A chlorinated hydrocarbon compound was analysed and found to consist of 24.24% carbon, 4.04% hydrogen and 71.72% chlorine. From another experiment the molecular mass was found to be 99 g.mol ${}^{-1}$ . Deduce the empirical and molecular formula.

## Molar volumes of gases

It is possible to calculate the volume of one mole of gas at STP using what we now know about gases.

1. Write down the ideal gas equation pV = nRT, therefore V = $\frac{nRT}{p}$
2. Record the values that you know, making sure that they are in SI units You know that the gas is under STP conditions. These are as follows: p = 101.3 kPa = 101300 Pan = 1 mole R = 8.31 J.K ${}^{-1}$ .mol ${}^{-1}$ T = 273 K
3. Substitute these values into the original equation.
$V=\frac{nRT}{p}$
$V=\frac{1mol×8.31J.{K}^{-1}.mo{l}^{-1}×273K}{101300Pa}$
4. Calculate the volume of 1 mole of gas under these conditions The volume of 1 mole of gas at STP is 22.4 $×$ 10 ${}^{-3}$ m ${}^{3}$ = 22.4 dm ${}^{3}$ .

examples of soluble substances
The most obvious example of it.. it salt and water where by salt is a solute because it dissolve in water (solvent)... Sugar and rice are example of insoluble because they do not dissolve
Olwethu
a 400 gram package lying on a horizontal surface is aytached to a horizontal string which passes over a smooth pulley.when a mass of 200gram is attached to the other end of the string , the package is on the point of moving . calculate the weight of the package
Assuming zero acceleration then the weight is w= mg which 0.4x9.8 = 3.92N. If you looking for nett F and acceleration in the system then that requires horizontal force and vertical force downwards subtracting upward force. There's no frictional force.
Martin
according to newton's third law of motion,the reaction force to the weight of a book lying on a table is the?
the measure of the refractive power of a medium is called what?
Reality-KiiD
how may i able to understand
what you what to understand
Nka
Try to learn physics language. In other words. If you read magnetic field, to you you have to feel and know what that means to you. If you've played with magnets and put them close, then hold them apart, the force pushing or pulling them is the magnetic force and the area around is th magnetic field
Martin
I've run out of space but begin keeping your own dictionary on physical science. The same would apply to sound, waves, force, acceleration, motion, etc
Martin
definition of an atom
How to calculate normal force
how do chemical bond
what is resources
What Is A Vector?
A vector is a force with both magnitude and direction
PARTY
what is reduction
reduction is the gain of electrons of an atom ; compound or molecule. reduction is also the decrease of the oxidation number of an atom ; compound / molecule.
Omphile
state Newton's second law of motion
Mthobisi
when resultant or net force act on an object an object will accelerate at the direction of the force at an acceleration directly proportional and inversely proportional to the mass
smosh
definition of Monochromatic light
Zolani
what's the difference between reducing and oxidising agent.and. what's the difference oxidising and oxidation number?
black
In Physics, monochromatic describes light that has the same wavelength so it's one colour
ocean_
What does Redox reactins mean?
Reaction in which there is transfer of electrons between elements
sibahle
how to calculate a vector
which rules do you have to follow to identify oxidation numbers
there are 7 of them > look them up on siyavula textbook physics
Omphile
Where can I get a Siyavula textbook?
Thimna
if possibly ther was a way to send a document I told hv sent it to you
Mhleli
Tatenda
Why do vectors have arrows?
vector quantity has both direction and magnitude so arrows illustrate the direction of the vector
Mhleli   By  By Anonymous User By By Rhodes  By David Martin By