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 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’.

HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l) 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.