2.3. Formulas
A subsection of Chemistry, 9701, through 2. Atoms, molecules and stoichiometry
Listing 10 of 88 questions
Combustion data can be used to calculate the empirical formula, molecular formula and relative molecular mass of many organic compounds. Define the term relative molecular mass. T is an alcohol, CxHyO. A gaseous sample of T occupied a volume of 20 cm3 at 120 °C and 100 kPa. The sample was completely burned in 200 cm3 of oxygen (an excess). The final volume, measured under the same conditions as the gaseous sample, was 250 cm3. Under these conditions, all water present is vaporised. Removal of the watervapour from the gaseous mixture decreased the volume to 170 cm3. Treating the remaining gaseous mixture with concentrated alkali, to absorb carbon dioxide, decreased the volume to 110 cm3. The equation for the complete combustion of T can be represented as shown. CxHyO + zO2 xCO2 + y 2H2O Use the data given to calculate the value of x. x = Use the data given to calculate the value of y. y = If you were unable to calculate values for x and y then use x = 4 and y = 10 for the remaining parts of this question. These are not the correct values. Complete the equation for the complete combustion of the alcohol, T. + O2 CO2 + H2O Give the skeletal formulae for two possible structures of T. Name each alcohol. Use the general gas equation to calculate the mass of T present in the original 20 cm3 gaseous sample, which was measured at 120 °C and 100 kPa. Give your answer to three significant figures. Show your working. mass = g
9701_s17_qp_21
THEORY
2017
Paper 2, Variant 1
View
Combustion data can be used to calculate the empirical formula, molecular formula and relative molecular mass of many organic compounds. Combustion data cannot distinguish between different structural isomers. Define the term structural isomers. P is a hydrocarbon, CxHy. A gaseous sample of P occupied a volume of 25 cm3 at 37 °C and 100 kPa. The sample was completely burned in 200 cm3 of oxygen (an excess). The final volume, measured under the same conditions as the gaseous sample (so that the water produced is liquid and its volume can be ignored), was 150 cm3. Treating the remaining gaseous mixture with concentrated alkali, to absorb carbondioxide, decreased the volume to 50 cm3. The equation for the complete combustion of P can be represented as shown. CxHy + (x + y 4)O2 xCO2 + y 2H2O Use the data given to calculate the value of x. x = Use the data given to calculate the value of (x + y 4). (x + y 4) = If you were unable to calculate values in and then use the data in this box for the remaining parts of this question. These are not the correct values. x = 6 (x + y 4) = 9 Give the molecular formula and the empirical formula of P. molecular formula of P empirical formula of P P is unbranched. Give the skeletal formulae for two possible structures of P that are positional isomers of each other. Use the general gas equation to calculate the mass of P present in the original 25 cm3 gaseous sample, which was measured at 37 °C and 100 kPa. Give your answer to three significant figures. mass = g
9701_s17_qp_23
THEORY
2017
Paper 2, Variant 3
View
A number of isomers with the formula Cr(H2O)6Cl 3 exist. Their general formula is [Cr(H2O)6-nCl n]Cl 3-n.nH2O. Each isomer contains a six co-ordinated Cr(ion in an octahedral complex. Water molecules not directly bonded with the Cr atom are held in the crystal lattice as water of crystallisation. The Cr–Cl bond is not easily broken and so chloride bonded with the Cr(ion in the complex does not react. 1.00 g samples of three of the isomers, A, B and C, were dissolved in separate samples of water. An excess of AgNO3was added to each and the mass of AgCl formed was measured. Ag++ Cl –AgCl The number of moles of AgCl formed was calculated. The table shows the results. isomer moles of AgCl formed from 1.00 g of isomer A 3.75 × 10–3 B 7.50 × 10–3 C 1.13 × 10–2 Calculate the Mr of Cr(H2O)6Cl 3. Mr Cr(H2O)6Cl 3 = Use the data in the table above to calculate the value of n for each of the isomers, A, B and C. Complete the table below with the values of n and the molecular formula of each isomer, in the style of the general formula given above. Show your working for at least one calculation of n. isomer n molecular formula A B C Two isomers have the same shape and their formula is Ni(R3P)2(CN)2, where R = CH3. Only one of these isomers has a dipole moment. Name the type of isomerism shown by Ni(R3P)2(CN)2. Draw structures of these two isomers. isomer 1 isomer 2 State which isomer has a dipole moment. Explain your answer.
9701_s17_qp_42
THEORY
2017
Paper 4, Variant 2
View
Many naturally occurring esters are used as flavourings in food. The structure of ester V is shown. O O V Name V. V reacts with a reagent to form a salt of a carboxylic acid and an alcohol. Identify a reagent that could be used in this reaction. Draw the displayed formula of the alcohol made during this reaction.  State one other possible use for V, apart from as a food flavouring. Ester W is made up of 54.5% carbon, 9.1% hydrogen and 36.4% oxygen. Calculate the empirical formula of W.  State what additional information is required to determine the molecular formula of W. Compounds X, Y and Z are shown. They all have the same molecular formula. O O X OH O O O Y Z Deduce the molecular formula of X, Y and Z. In three experiments, sodium is added to separate samples of X, Y and Z. Complete the table to show the observations for each of these three experiments. Ignore any temperature changes which may occur. experiment observations Na + X Na + Y Na + Z  Sodium carbonate solution reacts with methanoic acid. Write the equation for this reaction. 
9701_s19_qp_22
THEORY
2019
Paper 2, Variant 2
View
Questions Discovered
88