TOPIC E
CONSERVATION OF ATOMS
AND MASS
• The law of conversation of mass is a fundamental part of
chemistry.
• You will need to relate this idea
• as symbolic representations (equations), and
• As particulate representations (drawings).
• You might also need to perform calculations involving
moles and masses when analyzing experimental data.
• Solid carbon is known to react with oxygen to produce carbon dioxide.
If a mass of 1.2 g of carbon is burned in oxygen, 4.4 g of carbon
dioxide is found to form. How many grams of oxygen gas reacted with
the carbon?
• Which, if any, of the following representations of hydrogen gas
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•
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•
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burning in oxygen gas to form water, accurately expresses the law of
conservation of mass? In each case, explain your answer.
(a) H2 + O2  H2O
(b) H2 + 2O2  2H2O
(c) 2H2 + O2  2H2O
(d) H2 + H2 + O2  H2O + H2O
More …
• (e)
• (f)
Chemical equations, molar ratios and
analysis of analytes
• Use of moles is a key part of chemistry
• if we know the number of moles of a substance and we have a
balanced chemical equation
•  We can calculate the moles of another substance present in the
equation.
• Steps used:
• 1. Write a correct, balanced equation.
• 2. Find the number of moles present for one substance.
• 3. Use the stoichiometric coefficients* (conversion factor) in the
equation to find the number of moles of the unknown substance.
• 4. Find the number of moles for the unknown substance.
Gravimetric analysis and moles
• Gravimetric analysis involves the addition of a substance
to an aqueous solution to cause the formation of a solid.
• The substance that is added is specifically chosen to
react with the analyte (species undergoing analysis).
• For example, addition of silver ions to a solution that
contains chloride ions will result in the formation of a
precipitate of silver chloride, according to the balanced
equation below.
• Cl-(aq) + Ag+(aq)  AgCl(s)
• When no more precipitate forms
• the analyte (chloride ions) has been consumed, and
• the stoichiometric molar ratio has been reached.
• The solid is then separated from the solution by filtration
and subsequent drying.
• A solid of unknown composition contains some chloride ions.
A 0.182 g sample of the solid is dissolved in water and the chloride
ions dissolve to produce an aqueous solution. The solution has a large
amount of aqueous silver ions added to it until no more solid can be
formed.
• After filtering and drying, it is found that 0.287 g of solid are produced
in the reaction.
•
• (a) Identify the solid formed.
• (b) Calculate the moles of the solid formed.
• (c) Calculate the moles of chloride ions present in the original sample.
• (d) Calculate the mass of chloride ions present in the original sample.
• (e) Calculate the mass percentage of chloride ions in the original
sample.
• (f) What would the effect on the value calculated in (e) be, if the
precipitate were only partially dried? Explain your answer.
Titrations (volumetric analysis) and moles
• Chemical reactions are often carried out between
substances that are in solution.
• The concentration of a solution can be measured in:
• grams of the solute (solid) dissolved in a volume of solution.
• Or number of moles of the solute in a particular volume of solution.
• ( units = mol/L or mol L-1 called molarity (M)). M = mols / vol. (L)
• And mols = M  V
• Example:
• A solution has a concentration of 0.250 mol L-1 will have 0.250
moles of solute dissolved in 1.0 L of solution and can be referred to
as ‘0.250 M solution’ or a ‘0.250 ‘molar’ solution’.
Titration
• Titration is the experimental method of analysis that uses
concentrations of solutions.
• If we know a balanced chemical equation we:
• can calculate the moles of one substance,
• then then determine the moles of other substances
• And use that data to calculate an unknown concentration.
• As in gravimetric analysis, we need to use a substance that
specifically reacts with the analyte,
• For example addition of a solution of hydrochloric acid to a solution of
sodium hydroxide, will result in the formation of sodium chloride plus
water according to the balanced equation below.
• HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
• Since there is no solid formed, like in gravimetric analysis, we
have to have another way of determining that the analyte has
been totally consumed
• We accomplish this by using an indicator that changes color at
the equivalence point (the point at which the stoichiometric
molar ratio has been achieved).
• The observable event that occurs at the equivalence point is
called the end point.
• Hydroxides can be used to neutralize acids.
• It is found that an indicator changes color at the precise
moment that 44.0 mL of NaOH has been added to 25.0 mL of
2.00 mol L-1 HCI in a titration.
• Use this date to calculate the concentration of NaOH.