Chemistry, carbon-12 scale

1. An atom is composed of 8 electrons, 8 protons and 9 neutrons. This atom has a mass, on the carbon-12 scale, of approximately
a. 8
b. 16
c. 17
d. 25
What is carbon-12 scale? Why?

Carbon-12 scale is a scale of relative mass that is nowadays used. In carbon-12 scale, the relative mass of a carbon-12 atom (an atom that contains 6 protons, 6 neutrons and 6 electrons) is defined as exactly 12.

In carbon-12 scale, the relative mass of a proton and that of a neutron are both approximately equal to 1, and that of an electron is negligible. Hence, the relative mass of an atom is approximately equal to the sum of number of protons and that of neutrons.

Hence, the relative mass of this atom ≒ 8 + 9 = 17


2. The atom referred to is an isotope of
a. oxygen
b. fluorine
c. sulfur
d. chlorine

Atomic number is equal to the number of protons. From the Periodic Table, an atom of relative atomic mass 8 is oxygen.


3. In a chemical reaction 6 gram of sodium sulfate reacts with exactly 8 gram of barum chloride to produce 4 gram of sodium chloride and some barium sulfate
sodium sulfate(6 grams)+barium chloride(8 grams) -> sodium chloride(4 grams)+barium sulfate(? gram)
a.6
b.8
c.10
d.14
why?

According to the law of mass conservation:
Total mass of reactants = Total mass of products
(6 g) + (8 g) = (4 g) + Mass of barium sulfate
Hence, Mass of barium sulfate = (6 + 8 - 4)g = 10 g

kalvindeutsch 是錯的
首先，重量是不會消失
第二，就算用mole計，最後也要乘份子的molecular mass

ans for question 3 should be b

we need to consider which is the limiting agent
by calculation, BaCl2 is the limiting agent
some Na2SO4 would be left

for quicker solution
juz find out the no of mole of NaCl formed
no of mole of NaCl : no of mole of BaSO4 is 2:1
then mass of BaSO4 can be calculated

2010-03-03 21:51:48 補充：
我並沒有說重量會消失
我用no of mole計完後當然有乘埋molecular mass
如果我係錯既話
咁我就有一個疑問
6g Na2SO4既no of mole等於8g BaCl2既no of mole嗎?
如果唔係, 咁一定其中一個係limiting agent
reaction後其中一隻chemical species一定會有剩
唔係淨係得出product side