TOPIC ECONSERVATION 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 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.