STPM化学专区

數學神童

arrange the acids in the order of increasing acidity and explain why
ethanoic acid, benzoic acid, 4-nitratebenzoic acid and 4-hydroxylbenzoic acid

vick5821

1.If a solution of Na2SO4 is added dropwise to a 0.01M Ag2SO4 solution, what is the minimum concentration of SO4 ^2- is necessary to begin precipitate? (Ksp Ag2SO4 = 1.7 x 10^-5)

2.The solubility of benzoic acid,C6H5COOH in water is 5g L-1. In a food experiment, 2g of benzoic acid was added into 500ml of water as additive.
Calculate the pH of the solution. [Ka = 6.5 x 10^-5]

Can anyone help here?? thanks

silent91

arrange the acids in the order of increasing acidity and explain why
ethanoic acid, benzoic acid, 4 ...
數學神童 发表于 20-9-2010 10:31 PM

  try to think bout the electron donating gp & electron withdrawing group...
acidity of molecules refer to ability to release H+~
so, after tat, den u can find out dy~

vick5821

500 cm3 of 0.1 M solution of MgCl2 is added to 500 cm3 of 0.2M solution of Na2CO3 . Calculate the maximum mass of MGCO3 precipitate. Ksp MgCO3 = 1.0 x 10^-5

Please help..I am confused!!!

silent91

500 cm3 of 0.1 M solution of MgCl2 is added to 500 cm3 of 0.2M solution of Na2CO3 . Calculate the ma ...
vick5821 发表于 30-9-2010 11:02 PM

0.56g???? 不是是很确定，所以没写solution~

vick5821

nope.....................

數學神童

ammonia is very soluble in water.In room temperature,100cm^3 of its saturated solution contains 33g of ammonia.
a)calculate the concentration of a saturated solution of ammonia in moldm^-3

silent91

ammonia is very soluble in water.In room temperature,100cm^3 of its saturated solution contains 33g  ...
數學神童 发表于 24-10-2010 10:19 PM

    no of mole of NH3 in water = mass/rmm
                                          = 33/17
                                          = 1.9 mol

   concentration of saturated solution = 1.9/ (100cm^3 X 10^-3)
                                                    = 19 mol dm^-3
  
   感觉好像有问题。。。确定是３３g? 3.3g好像比较logic

    PS: 有错请纠正。

vUwBuC

Glass P - 500cm3,Nitrogen gas, 10Pa
    Glass Q - 200cm3,Helium gas,5Pa
JianWen 发表于 13-8-2010 07:57 PM

I did this kind of question before but now I almost forget
The way I do is ..

P1 = (500 x 10)/700 = 7.142
P2 = (200 x 5)/700 = 1.428
Total Pressure = P1 + P2
                      =  8.57 (round off = 8.6)

where 700 = total volume
P1 & P2 = partial pressure

Don't know whether is correct.

vick5821

How to draw a energy cycle for to illustrate the enthalpy of formation of ethanol given the heat of combustion of ethanol? How the energy cycle being constructed?? Any one pls help? Weak in writing energy cycle.!!thanks

nkrealman

How to draw a energy cycle for to illustrate the enthalpy of formation of ethanol given the heat of  ...
vick5821 发表于 15-11-2010 10:27 PM

    only the enthalpy of combustion of ethanol is given?do u have the complete questions?
energy cycle 应该还会给其他的enthalpy才对

vick5821

The combustion of 4.6g of ethanol has evolved 136.7 kJ of heat at 25* and 1 atm.Calculate the standard enthalpy of formation of ethanol.Given enthalpy of formation of CO2 =-393.5KJ mol-1, enthalpy of formation of H2O = -285.8KJ mol-1.....So my ques is,, how the energy cycle looks like?

vick5821

The combustion of 4.6g of ethanol has evolved 136.7 kJ of heat at 25* and 1 atm.Calculate the standard enthalpy of formation of ethanol.Given enthalpy of formation of CO2 =-393.5KJ mol-1, enthalpy of formation of H2O = -285.8KJ mol-1.....So my ques is,, how the energy cycle looks like?

nkrealman

4.6g/46=0.1 mol of ethanol release 136.7kj
1 mol of ethanol release 1367 kj.(all in standard conditions)

so the standard enthaply change of combustion of ethanol is 1367 kj per mol.

by writing out all the equations and cut off to form the equations of formation of ethanol...from there and calculate the enthalpy...this is safer way to calculate enthalpy instead forming energy cycle..

vick5821

Can I know how to construct the energy cycle??? Alternative ma..thanks

nkrealman

vick5821

Sometimes I am confused about the arrow..Can I draw the arrow all toward the ethanol??? mean the final product is ethanol..urs final product is CO2 and H20...so can???

vick5821

I mean I want a correct energy cycle..with a balance equation de

哈比人

50cm^3 of a mixture of methane and ethane are placed in a container. the mixture undergoes combustion in 200cm^3 of excess oxygen. the volume of the gases after combustion is 134cm^3. when the gases are passed through aqueous potassium hydroxide solution, the volume decreases to 52cm^3.

what is the volume of mehtane in the mixture ???
答案是 18cm^3.....可以给我解释吗？？

nkrealman

I mean I want a correct energy cycle..with a balance equation de
vick5821 发表于 18-11-2010 10:15 AM

   sry i seldom use this method..cant help u