papery Private
Messaged the following :
(Edit : Now I see wishboy has already answered his questions on
another thread. In future, everybody please post ur qns on the
forum and PM me to read the thread, instead of double-sending the
qns via PM.)
1. 20cm3 of
oxygen reacted with 20cm3 of carbon monoxide. what are the volumes
of the gases remaining, at the original temp and pressure?
The ans is oxygen-10cm3 carbon monoxide-0cm3 and
carbondioxide-30cm3. Why is this so?
Ans
:
CO + 1/2 O2
--> CO2
20 cm3 of CO
reacted with 10 cm3 of O2 to generate 20 cm3 of CO2, leaving behind
10 cm3 of excess O2. The given answer is wrong, ignore
it.
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2.what has a mass equal to that of one mole of water?
a. 24dm3 of water
b.one mole of steam
c. one molecule of water
d.2 moles of hydrogen molecules and one mole of oxygen
molecules.
why is the answer B?
Ans :
Steam = gaseous water. The sample mass of a species (eg. water)
does not change with its state. 1 mole of water has 18g mass
whether it is liquid or gaseous or solid (of course, when solid or
gaseous it's harder to measure the water alone, but if it can be
done, the mass will be exactly the same).
-------------------
3. What is the mass of aluminium in 204g of aluminium
oxide,al2o3(numbers are in sub script)
a.26g
b.27g
c.54g
d.108g
how do i get to the answer d?
Answer :
Moles of Al2O3 = 204 / (27x2 + 16x3) = 2 moles. In 2 moles of
Al2O3, 4 moles of Al metal is present. Hence 4 x 27 = 108g
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4. What do both an atom and an ion of the isotope of sodium
contain?
the ans is 12 neutrons. I uds that the neutron for the atom is 12
but for the isotope why is it 12 as well, i thought isotopes have
diff number of neutrons?
Ans :
The correct definition of isotope (the definition taught in
schools "Isotopes are atoms..." is slightly wrong, the correct
definition) is "Different isotopes of an element have
atoms and/or ions with the same number of protons, but different
number of neutrons". In this question, we're referring to a
single particular isotope of sodium, which means the number of
neutrons must be the same, whether it's an atom or an ion of this
particular isotope.
-------------------
5. I have another question which have a diagram but i can't
display it here. Any idea how i can show you? I have it in my
laptop.
Ans :
Upload it to some free file sharing website, then include the
image's url in your SgForum post.