Unit 2: Chapter 3 - How Much?

Question 1

Mass of Atoms

Answer 1

Subatomic Particles and their Relative Mass:
Protons (+) - 1
Electrons (-) - 0 (negligible)
Neutrons (no charge) - 1

Isotopes are atoms that are the same elements that have the same number of protons but have a different amount of neutrons.
Isotopes give different properties so for example, tritium, a hydrogen isotope is radioactive.

Question 2

Mass of atoms and Moles

Answer 2

Relative Atomic Masses (RAM or Ar) are used because calculating the real masses of atoms would be impractical.

RAM is the Mass number of an element (Neutrons and Protons)

RELATIVE FORMULA MASSES is the mass of COMPOUNDS, such as NaCl (58.5).

MOLES is the RAM/g.
- One mole of any substance is it relative formula mass, in grams. E.g. 1 mole of carbon is 12g.

Question 3

Percentages and Formula

Answer 3

EMPIRICAL FORMULA FORMULA:
Mass/ RAM = Moles

WORKING OUT PERCENTAGE OF AN ELEMENT IN A COMPOUND:

RAM of element / RFM of compound

EMPIRICAL FORMULA OF A COMPOUND FROM ITS PERCENTAGE COMPOSITION:
e.g.

A hydrocarbon is 75% carbon and 25% hydrogen by mass. What is the empirical formula?

Imagine we have 100g of the compound.
75g of carbon and 25g of hydrogen.

Mass/ RAM = Moles , therefore

Carbon: 75/12 = 6.25 moles.
Hydrogen: 25/1 = 25 moles.

6. 25(C) :25(H)
   1: 4 ratio so the empirical formula is:

CH4.

Question 4

Equations and Calculations

Answer 4

HOW MUCH CALCULATION:

If we have a solution containing 100g of sodium hydroxide, how much chlorine gas do we need to convert it to BLEACH?

NaOH (RFM): 40
Cl2 (RFM): 71

Mass/RAM = MOLES (NaOH) 
100/40 = 2.5 MOLES (NaOH)

2.5 / 2 = 1.25 chlorine moles (because it was in Cl2 form)

Moles x RFM = Mass
1.25 x 71 = 88.75g to react with 100g of sodium hydroxide

Question 5

Yield of a Chemical Reaction

% Yield equation?
Give 5 reasons why a chemical reaction may not give 100% yield.

What is sustainable Production?

Answer 5

% Yield = Amount of product produced / Maximum of product possible

Very few chemical reactions have a yield of 100% because:

• Reversible reaction
• Some reactants give unexpected products
• Some of the product may be lost in handling or left behind in apparatus.
• Reactants may not be 100% pure.
• Chemical reactions produce unwanted products and may be difficult to separate the product mixture.

SUSTAINABLE PRODUCTION?

Ideally, a company wants a high yield. Making a product more efficiently means making less waste.

Chemical engineers design plants in the safest, most economical way possible. It should be as efficient as possible so it does not take a toll on the environment with pollutants such as CO2 but also conserves limited resources.

Question 6

Reversible Reactions

Answer 6

Reactions which cause the products to revert back to reactants are called REVERSIBLE REACTIONS.

To show a reversible reaction, we show 2 arrows in both directions.

EXAMPLE (1) Litmus Paper

Neutralizing alkaline solutions with acid. Indicators react in acids to form color compounds. They also react in alkalis to form a different color compound.

HLit H(+) + Lit(-)

The reaction an go either way depending on if you add more acid or alkali.

EXAMPLE (2) Ammonium Chloride

Ammonium Chloride (NH4Cl) breaks down upon heating. It forms ammonia gas (NH3) and hydrogen chloride gas (HCl). This is an example of THERMAL DECOMPOSITION.

When the two gases rise to the top of the test tube, they react back together and reform ammonium chloride.

Question 7

Analyzing Substances

Answer 7

Food must be analyzed to see what additives are used so that they are safe. Additives are approved by being given E numbers.

DETECTING ADDITIVES

We can detect additives through CHROMATOGRAPHY. There are two ways to do chromatography:

• Paper Chromatography (Traditional)
• Gas Chromatography (Modern)

Paper chromatography is used because:

• Some compounds dissolve better than other solvents. Their solubility depends on how far they travel.
• Cheap
• Relatively quick
• Easy to use

Gas chromatography is used because:

• It is highly accurate and precise
• They enable very small quantities to be sampled
• Quick
• Require less people for analysis

but they:

• give results that can only be interpreted by comparison with other data of known substances.
• Are expensive
• take special training to use

Question 8

Instrumental Analysis

Answer 8

Gas chromatography (How do you use it?)

• Separate compounds by vaporizing
• Mixture is moved through a coiled column
• Substances with high attraction with the column give longer RETENTION TIMES.
• Substances with less retention time leave first.
• Separated compounds are then passed through to a Mass spectrometer to identify elements and compounds.
• The analysis between different substances must be done in the same conditions to compare retention times.

MASS SPECTROMETER

To ensure we are identifying the unknown substances in gas chromatography, we use a MASS SPECTROMETER.

The Mass Spectrometer measure RFM (Relative Formula Mass) of a compound to identify it.

It is very accurate
It only requires very small samples of substances to analyze.