| Date | May 2017 | Marks available | 1 | Reference code | 17M.2.sl.TZ2.1 |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Show | Question number | 1 | Adapted from | N/A |
Question
There are many oxides of silver with the formula AgxOy. All of them decompose into their elements when heated strongly.
After heating 3.760 g of a silver oxide 3.275 g of silver remained. Determine the empirical formula of AgxOy.
Suggest why the final mass of solid obtained by heating 3.760 g of AgxOy may be greater than 3.275 g giving one design improvement for your proposed suggestion. Ignore any possible errors in the weighing procedure.
Naturally occurring silver is composed of two stable isotopes, 107Ag and 109Ag.
The relative atomic mass of silver is 107.87. Show that isotope 107Ag is more abundant.
Some oxides of period 3, such as Na2O and P4O10, react with water. A spatula measure of each oxide was added to a separate 100 cm3 flask containing distilled water and a few drops of bromothymol blue indicator.
The indicator is listed in section 22 of the data booklet.
Deduce the colour of the resulting solution and the chemical formula of the product formed after reaction with water for each oxide.
Explain the electrical conductivity of molten Na2O and P4O10.
Outline the model of electron configuration deduced from the hydrogen line emission spectrum (Bohr’s model).
Markscheme
n(Ag) = «\(\frac{{3.275{\text{ g}}}}{{107.87{\text{ g}}\,{\text{mol}}}} = \)» 0.03036 «mol»
AND
n(O) = «\(\frac{{3.760{\text{ g}} - 3.275{\text{ g}}}}{{16.00{\text{ g}}\,{\text{mo}}{{\text{l}}^{ - 1}}}} = \frac{{0.485}}{{16.00}} = \)» 0.03031 «mol»
«\(\frac{{0.03036}}{{0.03031}} \approx 1\) / ratio of Ag to O approximately 1 : 1, so»
AgO
Accept other valid methods for M1.
Award [1 max] for correct empirical formula if method not shown.
[2 marks]
temperature too low
OR
heating time too short
OR
oxide not decomposed completely
heat sample to constant mass «for three or more trials»
Accept “not heated strongly enough”.
If M1 as per markscheme, M2 can only be awarded for constant mass technique.
Accept "soot deposition" (M1) and any suitable way to reduce it (for M2).
Accept "absorbs moisture from atmosphere" (M1) and "cool in dessicator" (M2).
Award [1 max] for reference to impurity AND design improvement.
[2 marks]
Ar closer to 107/less than 108 «so more 107Ag»
OR
Ar less than the average of (107 + 109) «so more 107Ag»
Accept calculations that gives greater than 50% 107Ag.
[1 mark]
Do not accept name for the products.
Accept “Na+ + OH–” for NaOH.
Ignore coefficients in front of formula.
[3 marks]
«molten» Na2O has mobile ions/charged particles AND conducts electricity
«molten» P4O10 does not have mobile ions/charged particles AND does not conduct electricity/is poor conductor of electricity
Do not award marks without concept of mobile charges being present.
Award [1 max] if type of bonding or electrical conductivity correctly identified in each compound.
Do not accept answers based on electrons.
Award [1 max] if reference made to solution.
[2 marks]
electrons in discrete/specific/certain/different shells/energy levels
energy levels converge/get closer together at higher energies
OR
energy levels converge with distance from the nucleus
Accept appropriate diagram for M1, M2 or both.
Do not give marks for answers that refer to the lines in the spectrum.
[2 marks]
