chem tys questions

anpanman SG

1. Explain why fermentation alone cannot make pure ethanol.

My ans: fermentation of sugar produces only a dilute solution of ethanol (about 15%) . when alchol content exceeds this value, yeast dies causing fermentation to stop.

2. the table below shows some info about an homogolous series of carbon compounds called ethers

name boiling point

methoxymethane -24.8

methoxyethane 7

methoxypropane 38.8

methoxybutane 70.3

the ethers are all liquids at r.t.p true/false my answer is false since methoxymethane is already a GAS at r.t.p. the answer in bold is given by tys

the ethers are saturated compounds true/false got this quesiton correct but how do i know they are saturated compounds?

3. Mendeleev devised the modern periodic table. he put the elements in the same group if their properties were similar. one of the properties he used was the formula of the oxides of the elements

when group1 metals tarnish in air, different types of oxides are formed.

element oxides formed formula

lithim lithim oxide Li2O

sodium sodium oxide Na2O

sodium peroxide Na2O2

potassium potassium superoxide KO2

rubidium rubidium superoxide RbO2

(ai) She put all these elemts in group 1. Do the formulae of the oxides given in the table show clearly that all the elements belong in the same group? explain your answer.

(aii) do the formulae of the oxides given in the table show a trend down the group? expain your anwer.

don't really know how to answer the 2 questions.

4. a mixture of white solids is treated with an excess of dilute HCl. A colourless gas is evolved and some but not all of the mixture dissolves, what could the mixture contian?

CaCO3 and BaSO4 OR

Ca(OH)2 and MgCO3

the answer is CaCO3 and BaSO4. but i don't udnerstand. the products given would be CaCl2, CO2, H2O and also BaCl2 if that's the case. All the products are soluble and with excess acid, i see no reason why NOT all the mixture dissolves. Ca(OH)2 and MgCO3 is also a possible answer in this case.

5. lastly, if i want to calculate how many molcules are there in 1 mol of ethnaol.

do i use this formula 1 6.02 X10^23? (or is this used to find no. of atoms?)

Thanks!

UltimaOnline SG

Q5. The word “mole” refers to 6.02×10^{23. So 1 mole of
ethanol contains 6.02×10}23 of ethanol molecules, and hence 9×6.02×10^23 atoms.

Q4. BaSO4(s) is insoluble, and adding HCl won’t dissolve it. Unlike adding excess NH3 to Cu(OH)2(s), which would dissolve the Cu(OH)2 ppt since positition of equilibrium shifts as Cu2+ is used up to form the soluble complex ion tetraaminediaquocopper(II) ion.

Q3. Oxides are stable, but peroxides and superoxides are easily polarized and distorted by cations of higher charge densities, and hence peroxides and oxides are only commonly present with cations of lower charge densities.

Q2. An ether functional group simply means R-O-R, and the R groups may or may not be saturated (ie. it’s a different matter). An unsaturated compound refers to double or triple bonds present between two adjacent C atoms. In the list of ethers given, none of them are unsaturated.

Q1. You’re correct. No matter what the yield at varying temperatures would be, repeated fractional distillation would need to be carried out. Wikipedia industrial alcoholic fermentation for more info.

lamebiskit SG

the ethers are all liquids at r.t.p true/false my answer is false since methoxymethane is already a GAS at r.t.p. the answer in bold is given by tys

-you are correct, maybe tys screw up or left out other infomation.

the ethers are saturated compounds true/false got this quesiton correct but how do i know they are saturated compounds?

-saturated compounds means only single bonds exists in the molecule. unsaturated compounds have presence of double, triple bonds.

(ai) She put all these elemts in group 1. Do the formulae of the oxides given in the table show clearly that all the elements belong in the same group? explain your answer.

-yes. the elements in the same group share the same ionic state, (e.g. )Na+, K+ are grp1) thus the basic oxides show it is the same group. peroxide O₂²⁻ is a compound containing an oxygen-oxygen single bond, thus not considered since these are complex molecules which are unstable in nature, same for superoxides O₂^−. In all these compounds the ionic state of the group 1 elements remains at 1+.

(aii) do the formulae of the oxides given in the table show a trend down the group? expain your anwer.

- the only trend i can see from the info provided is that going down the group, the elements are more readily available to form complex, chemically unstable compounds like superoxides. This could be due to the increase in atomic size and thus electronic composition.

4. a mixture of white solids is treated with an excess of dilute HCl. A colourless gas is evolved and some but not all of the mixture dissolves, what could the mixture contian?

CaCO3 and BaSO4 OR

Ca(OH)2 and MgCO3

-you are partially correct regarding CaCl2, CO2, H2O. Remember to check back to ur lab experiments, BaSO4 is insoluble in excess dilute acid since the acid is not powerful enough to oxidise BaSO4. BaSO4 is only soulble in concentrated sulphuric acid.

5. lastly, if i want to calculate how many molcules are there in 1 mol of ethnaol.

do i use this formula 1 6.02 X10^23? (or is this used to find no. of atoms?)

-yes. 1 mol of anything contains avogardo's number of molecules, atoms.

anpanman SG

Thanks UltimaOnline and lamebiskit, now i can understand the question and can answer them better!