chem 問題.....會考

hahahaha

2008-04-01 1:30 am

i do some exercise but i do not know how to calculate..

1. 3g of magnesium ribbon is added into 100cm^3 of 2M magnesium nitrate solution . Calculate the molarity of magnesium nitrate after 50cm^3 of 1.5M HNO3(aq) is added into the solution.

the answer is 1.58M why?

2.Which powder needs the shortest time to react with dilute sulphuric acid completely?
A.7g of calcium
B:8g of magnesium
C: 8g of zinc
D:4g of copper

the answer is B WHY?
我知道可能你會話多d g react緊係快d.但係calcium 係higher ecs架wo.唔係快d既咩.有咩方法計到.係唔係有方法計?

仲有一題.
A copper plate and a zinc plate are placed in a beaker containing dilute supluric acid .The plates of copper and zinc touch each other .
THis is a mc question

I do not know why gas bubble are formed on the surface of zinc plate is incorrect?

得閒順便講埋二個2 hydrocarbon 一樣general formula既但係d位有小小唔一樣..點解佢地boiling point 唔係the same

thx..我就黎會考.希望有好心人教我..><"

回答 (1)

[匿名]

2008-04-01 2:33 am

✔ 最佳答案

第一題我琴日先做完
no. of mole of Mg=1.3/24.3=0.053497942mol
no. of mole of HNO3=0.05x1.5=0.075mol
no. of mole of Mg(NO3)2 formed=0.0375mol
Molarity=[0.0375+(2x0.1)]/[0.05+0.1]=1.583333333M

第二題因為Ca同H2SO4 react to form CaSO4 "PPT"
which prevent further reaction

第三題既setup同一個chemical cell一樣只係冇左電線(external circuit)
即係會有redox reaction
zinc表面會有oxidation: Zn===>Zn^2+ + electron
電子流去copper表面並discharge H+ in sulphuric acid
reduction: H^+ + electron===> hydrogen gas
所以gas bubble係響copper表面而唔係zinc表面.

最後果題唔係好肯定,可能關於molecules之間既空間.例如but-1-ene同but-2-ene, d atoms "zip"係 molecules中間d空位既程度唔同令attraction唔同, 所以boiling point/melting point唔同.

希望幫到你,因為我都係應屆會考生^^

參考: 自己.

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