STPM 化学问题 马拉松！

Leong13

a buffer solution was prepared by dissolve 0.26 mol benzoic acid, C6H5COOH and 0.34mol sodium benzoate, C6H5COONa, in distilled water and made up to 800cm^3 solution.
Ka = 6.5 x 10^-5

a. calculate the pH of the buffer solution
b. calculate the  pH of the resulting solution when 100cm^3 of 0.05moldm^-3 HCl was added to 100cm^3 of the buffer solution.
c. calculate the pH of the resulting solution when 100cm^3 of 0.05moldm^-3 NaOH was added to 100 cm^3 of the buffer solution.

nikuang04

以下C6H5COOH写成acid，C6H5COONa写成salt（懒惰）
[acid]=0.26/0.800=0.325
[salt]=0.34/0.800=0.425
pH=-lg6.5X10^-5+ lg 0.425/0.325=4.30

b)number of moles of HCl=0.005
100cm3 contains 0.0325 of acid and 0.0425 of salt
salt react with HCl to procude acid
mole salt=0.0425-0.005=0.042
mole acid=0.033
pH=4.187+lg0.042/0.033=4.29

c)pH=4.187+lg0.0430/0.0320=4.32

不懂对不对？

Leong13

b 和 c 的答案不對
再試試吧。

chunwei1088

原帖由 nikuang04 于 5-9-2007 01:14 AM 发表
以下C6H5COOH写成acid，C6H5COONa写成salt（懒惰）
[acid]=0.26/0.800=0.325
[salt]=0.34/0.800=0.425
pH=-lg6.5X10^-5+ lg 0.425/0.325=4.30

b)number of moles of HCl=0.005
100cm3 contains 0.0325 o ...

方法应该对了。。。可是MINUS错了。。

nikuang04

原帖由 chunwei1088 于 5-9-2007 21:06 发表

方法应该对了。。。可是MINUS错了。。

原来是减错了
谢谢你


这样对了吗?
b)number of moles of HCl=0.005
100cm3 contains 0.0325 of acid and 0.0425 of salt
salt react with HCl to procude acid
mole salt=0.0425-0.005=0.0375
mole acid=0.0375
pH=4.187+lg1=4.187

c)pH=4.187+lg0.0475/0.0275=4.42

[ 本帖最后由 nikuang04 于 6-9-2007 12:38 AM 编辑 ]

Leong13

對了。

芋头糕01096

我是新来的…

Which of the following statements explains why copper conduct electricity when a potential difference is applied?

A Copper(II) ions move tothe cathode
B The crystal lattice breacks down.
C The atoms of copper become ionised.
D Electrons combine with copper(II) ions.
E Bonding electrons in the crystal lattice move.

jiko

不懂是A还是E涅。。。

我想我选E好了

芋头糕01096

Ans: E
In metallic Cu,metallic bond exist.
Mobile electron 'cloud' accounts for the e-conductivity.
In the presence of E potential,the electrons flow from (-) to(+)

ynnee

A gaseous hydride of nitrogen,x,contain 12.5%of hydrogen by mass.when 1 mole of x was heated strongly,x decompose to 3 mole of gaseous element .which of the following could be x?
(relative atomic mass:H=1 N=14)

A NH2             D N3H6
B NH3             E N4H8
C N2H4

ANS WHY?

jiko

是D吗？？？？

ynnee

answer is C?我做出来的答案是NH2?可是为什么变成N2H4?

jiko

这样的话~~~
我大概懂了！

N2H4------> N2 + 2 H2
total=3 mole of gas

solite

没题目了吗？？
上面的人

avada

我来帮忙出题
An organic compound H with relative molecular mass 107 steam-
distils at 10^5Pa and 368K.If the vapour pressure of water is 9.3x10^4Pa at 368K, calculate the mass of H found in 20g of the distillate.

jiko

是1.5g吗？？？？？？？？？？？？？？

huhuxboy

原帖由 avada 于 25-9-2007 09:40 PM 发表
我来帮忙出题
An organic compound H with relative molecular mass 107 steam-
distils at 10^5Pa and 368K.If the vapour pressure of water is 9.3x10^4Pa at 368K, calculate the mass of H found in 20g ...

let m(H) = y
  m(H)              P(H) x M(H)
———   = —————————
m(H2O)      P(H2O) x M(H2O)


y           (10^5-9.3x10^4)(107)
—   = ——————————
20-y      ( 9.3x10^4)(18)


y = 6.18g

不知道对不对。。。

rickykhoo

rickykhoo

partial pressure of H compound = (10^5)-(9.3x10^4) = 7000
mole fraction of H compound = 7000/(10^5) = 0.07

为什么人家一个equation就搞定了。。
我做到这么长。。
不怪得我考试会没时间啦。。

avada

答案是６。１８g 方法如 huhuxboy 和 rickykhoo 所说

下一题
The rate constant for the following reaction are 8.22x10^-4
at 333K and 3.7x10^-3 at 393K.What is the activation energy of   this reaction?