Chemistry: Post your doubts here!

[Rizwan Javed]

RCH=CH2
If a terminal carbon atom, like shown in red above, is bonded to two hydrogen atoms we get oxidation to a CO2 molecule.

 

[Abdullahassan_99]

First write the Kc expression for this reaction:
Kc = [HI]^2 / [I2]*[H2]

We know Kc = 60 at the specified temperature.

Assume a container of 1dm^3 for simplicity:
[H2] = 2
[I2] = 0.3
Kc = 60
[HI] = sqrt(Kc*[I2]*[H2]) = sqrt(60*0.3*2) = sqrt(36) = 6mol/dm^3

Thank you SO much

 

[Eugene99]

Reaction of aldehyde with Tollen's reagent makes silver mirror inside the test tube. But when I mixed Tollen's reagent with aldehyde, and heated it up, only black precipitate appeared, no silver mirror, why?

 

[Rizwan Javed]

Reaction of aldehyde with Tollen's reagent makes silver mirror inside the test tube. But when I mixed Tollen's reagent with aldehyde, and heated it up, only black precipitate appeared, no silver mirror, why?

First, a black ppt is formed, but if you shake the test-tube gently, you'll see that silver mirror.

 

[Eugene99]

First, a black ppt is formed, but if you shake the test-tube gently, you'll see that silver mirror.

it just forms a blackish water after mixing could my aldehyde be stale since I have it for almost a year, Tollen's reagent I just bought recently

 

[Rizwan Javed]

it just forms a blackish water after mixing could my aldehyde be stale since I have it for almost a year, Tollen's reagent I just bought recently

It doesn't make a difference, i guess. You can see a black ppt, it is an enough indication that aldehyde is present.

 

[qwertypoiu]

It doesn't make a difference, i guess. You can see a black ppt, it is an enough indication that aldehyde is present.

Do you know what compound is causing this black ppt?

 

[Rizwan Javed]

Do you know what compound is causing this black ppt?

I think that color is due to the formation of Ag (s).

 

[Bhaijan]

9701_w03_qp1
Ans is B
I dont understand how to solve this question
pls help!!!

 

[Mohammed Elatta]

View attachment 59786
9701_w03_qp1
Ans is B
I dont understand how to solve this question
pls help!!!

From the data given to us we know that 30% of the fertiliser mass is P2O5. Get the mass of the fertiliser.
In 100g of the fertiliser there is 30% of it P2O5 therefore mass of fertiliser is
(100/30) x 142= 473.3g
to know the percent by mass of phosporus in fertiliser calculate the mass of phosphorus as we have 2 atoms within each molecule of P205 in 1 mole the mass of phosporus is 31x2=62 calculate the percentage now
62x100/473.3= 13.1

 

[Bhaijan]

From the data given to us we know that 30% of the fertiliser mass is P2O5. Get the mass of the fertiliser.
In 100g of the fertiliser there is 30g P2O5 therefore mass of fertiliser is
(100/30) x 142= 473.3g
to know the percent by mass of phosporus in fertiliser calculate the mass of phosphorus as we have 2 atoms within each molecule of P205 in 1 mole the mass of phosporus is 31x2=62 calculate the percentage now
62x100/473.3= 13.1

wow thnks i didnt think to find the mass of fertilizer

 

[holoholo]

When preparing a solution of a fixed concentration from a given parent solution of concentration , say, 2.0 mol/dm3

A. We are required to prepare a solution of concentration of 1.0 mol/dm3 with volume 250 cm3 ( the concn cannot be greater than 2 !!!!!!!!!!!!!!)
a. Add 100 cm3 of the parent solution to a volumetric flask of marking at 250 cm3
b. Top off with water to the mark of 250 cm3. Use a wash bottle for this purpose to have greater control over the addition process


Shouldn't we add 125cm^3 of the parent solution not 100 cm^3 ?

 

[holoholo]

Also in this
When required to prepare 250 cm3 of 0.5 mol/dm3 solution of a crystal of Mr 50g

First realize that we need only 250 cm3, not 1 dm3
Now in 250 cm3 there will be: 50/4 = 12.5 g of the solid

Shouldn't it be 6.25 g ?

 

[qwertypoiu]

When preparing a solution of a fixed concentration from a given parent solution of concentration , say, 2.0 mol/dm3

A. We are required to prepare a solution of concentration of 1.0 mol/dm3 with volume 250 cm3 ( the concn cannot be greater than 2 !!!!!!!!!!!!!!)
a. Add 100 cm3 of the parent solution to a volumetric flask of marking at 250 cm3
b. Top off with water to the mark of 250 cm3. Use a wash bottle for this purpose to have greater control over the addition process


Shouldn't we add 125cm^3 of the parent solution not 100 cm^3 ?

If you want to prepare 250cm3 of 1mol/dm3 solution from 2mol/dm3 solution, you need to use 125 of it yes. Then 125 of water. So that the concentration is halved.

 

[qwertypoiu]

Also in this
When required to prepare 250 cm3 of 0.5 mol/dm3 solution of a crystal of Mr 50g

First realize that we need only 250 cm3, not 1 dm3
Now in 250 cm3 there will be: 50/4 = 12.5 g of the solid

Shouldn't it be 6.25 g ?

Yes correct. Because it's 0.5mol/dm3 not 1mol/dm3.


(PS you should not trust this source)

 

[Abdullahassan_99]

Can someone tell me why the answer is A?

 

[Rizwan Javed]

Can someone tell me why the answer is A?

Use the general gas equation to solve this question.
PV = nRT
Before and after the value is opened, the n (moles of the gas) will remain the same. So make n the subject of the equation:
n = PV/RT
Before opening, let the volume of M be V.
After opening the volume will be 4V. (Volume of M + 3 * Volume of M)

Now simply substitute the values, into the equation above for two situations: before the valve is opened and after the valve is opened. And put them equal:

((10^5)*V)/(8.31*(20+273)) = (P*4V)/(8.31*(100+273))

Solve it to find P (the final pressure)

You'll get the answer A.

 

[nowaisir10]

Thank you so much qwertypoiu

hey are you giving AS this may with variant 12 and 22?

 

[nowaisir10]

Can someone explain question 3iv) for me? I looked for the mark scheme but I couldn't understand it. http://studyguide.pk/Past Papers/CIE/International A And AS Level/9701 - Chemistry/9701_w07_qp_4.pdf[
do you have facebook Id because I want to make a group in facebook who will be giving AS this may with variant 12 and 22

 

[Saad the Paki]

hey are you giving AS this may with variant 12 and 22?

Nope I'm giving 11 and 21