Edexcel CHEMISTRY DOUBTS!!!!

[elemis]

Question 5

Obviously, a cation MUST polarise an anion.

Also, the LARGER the charge on the cation the smaller its radius, the greater the charge density and hence the larger the ability to polarise.

Since we must look for cations with the largest charge we can eliminate C and D.

A and B are left. However they both have the same charge..... what to DO  ?

Barium is lower in the group than Calcium, correct ? Hence, Barium's radius is larger, correct ? Therefore, its charge density is SMALLER than Calcium.

Hence, answer = B

[The Golden Girl =D]

Question 4

why does the one after it HAVE to have a 5 p orbital  


the d part I do get it

[The Golden Girl =D]

Question 5

Obviously, a cation MUST polarise an anion.

Also, the LARGER the charge on the cation the smaller its radius, the greater the charge density and hence the larger the ability to polarise.

Since we must look for cations with the largest charge we can eliminate C and D.

A and B are left. However they both have the same charge..... what to DO  ?

Barium is lower in the group than Calcium, correct ? Hence, Barium's radius is larger, correct ? Therefore, its charge density is SMALLER than Calcium.

Hence, answer = B

I saw it as Beryllium -.-

[elemis]

Question 9

0.9 * 2/3 = 0.6 moles

Since, moles = volume/24dm3

We get : 0.6 *24

From my workings you can see the answer is C

[elemis]

why does the one after it HAVE to have a 5 p orbital  


the d part I do get it

Wrong. BEFORE IT !!!

The electronic config of Neon is : 1s² 2s² 2p⁶

Therefore, the element BEFORE it (Fluorine) must be 1s² 2s² 2p⁵

Therefore it has 5 electrons in its P orbital.

[The Golden Girl =D]

Now I get it

But as for Q 9] I don't get why we multiply it by (2/3) ?

you forgot Q10 

[elemis]

You have to form an equation :

NaCl ------> Na⁺ + Cl^-

MgCl₂ -----> Mg²⁺ + 2Cl^-

Now basically you do : concentration*volume/1000

I'll let you figure it out now, since I've given you the hint

Make sure to consider the molar ratios

[elemis]

Now I get it

But as for Q 9] I don't get why we multiply it by (2/3) ?

you forgot Q10 

Consider it terms of a ratio :

2 moles of SO₃ / 3 moles of O₂ =  X moles of SO₃ / 0.9 moles of O₂

So you get 2/3 = x/0.9

x = 2/3 * 0.9

[The Golden Girl =D]

Now I understand them ALL

Thanks mate

[elemis]

Now I understand them ALL

Thanks mate

No worries

[Haaks18]

What does thermochemical equation means?!
What does "the bigger charge" means?! does it means the size of the charge

[Deadly_king]

What does thermochemical equation means?!
What does "the bigger charge" means?! does it means the size of the charge

In a thermochemical equation, we include the value of DH, the change in enthalpy for the reaction. For example, the equation below showing the combustion of hydrogen would be written as

2H₂(g)+ O₂(g) ----> 2H₂O(g), DH = -571.6 kJ

The sign and magnitude of the change in enthalpy tell us that this reaction is exothermic and gives off a lot of heat. Some rules for writing thermochemical equations are:

   1. The sign of DH indicates if the reaction is exothermic (DH negative) or endothermic (DH positive)
   2. The coefficients in front of the species represent number of moles
   3. The phases of each reactant and product must be indicated: the enthalpy of liquid water at 25^oC is different from gaseous water at the same temperature, for example
   4. The value of DH is computed with all reactants and products at 25^oC and 1 atm pressure unless otherwise indicated.

Here are some examples of chemical equations.

Bigger charge only means that the charge on the ion is greater.

Example : Na⁺ and Mg²⁺

Mg²⁺ has the bigger charge since it has a charge of 2 units while Na⁺ has only one.

[**RoRo**]

I've got a couple of MCQ's, I have the answers to them but I need an explanation as to how do you come to a conclusion to that answer.

1. A 5.00g sample of anhydrous Group II metal nitrate loses 3.29g in mass on strong heating. Which metal is present?
A) Magnesium
B) Calcium
C) Strontium
D) Barium
The answer is B) Calcium.
[I found a method to do this, but it's VERYYY time consuming, where I found the no. of moles of each of those 4 metal nitrates and the no. of moles of the metal oxides and calcium was the only one that had the same no. of moles [according to the mole ratio]!

2. The foul smell that skunks spray is due to the number of thiols, one of which is methanethiol CH₃SH, which burns as follows:
CH₃SH + 3O₂ ----> CO₂ + SO₂ + 2H₂O
A sample of 10cm³ of methanethiol was exploded with 60cm³ of oxygen.
What would be the final volume of the resultant mixture of gases when cooled to room temperature?
A) 20cm³
B) 30cm³
C) 50cm³
D) 70cm³
The answer is C) 50cm³

3. The amount of calcium ions in a sample of natural water can be determined by using an ion exchange column.
A 50cm³ sample of water containing dissolved calcium sulphate was passed through the ion exchange resin. Each calcium ion in the sample was exchanged for two hydrogen ions. The resulting acidic solution collected in the flask required 25cm³ of 1.0x10^-2mol dm^-3 Potassium hydroxide for complete neutralization.
What was the concentration of the calcium sulphate in the original sample?
A) 2.5x10^-3 mol dm^-3
B) 1.0x10^-2 mol dm^-3
C) 2.0x10^-2 mol dm^-3
D) 4.0x10^-2 mol dm^-3
The answer is A) 2.5x10^-3 mol dm^-3

& Thanks in advance!!

[Amelia]

Ans.1) - Can't think of a better way, myself. Sorry.


Ans.2) you get the answer by simply subtracting 10 from 60. The 10 cm³ of methanethiol is used up/exploded with 60cm³, so the final volume of the resultant mixture is 50.

Ans 3) - Are you sure the answer is A?, I'm getting it as C.
 Each calcium ion in the sample was exchanged for two hydrogen ions.
            Therefore, half the concentration of Potassium Hydroxide was required to neutralize the reaction.
            (1.0x10^-2)X2 = 2.0x10^-3

[**RoRo**]

Ans.1) - Can't think of a better way, myself. Sorry.


Ans.2) you get the answer by simply subtracting 10 from 60. The 10 cm³ of methanethiol is used up/exploded with 60cm³, so the final volume of the resultant mixture is 50.

Ans 3) - Are you sure the answer is A?, I'm getting it as C.
 Each calcium ion in the sample was exchanged for two hydrogen ions.
            Therefore, half the concentration of Potassium Hydroxide was required to neutralize the reaction.
            (1.0x10^-2)X2 = 2.0x10^-3

It's alright, I've contacted my teacher at school and she's clarified my doubts then!

For 1)
We know that 5g of the Metal nitrate gives 3.29 grams of the mass lost [i.e. nitrogen dioxide and oxygen], so we find the molar mass of the 2 gases which is 216g and then using ratio and proportion we find the molar mass of the metal nitrate which is 328g.

This molar mass is for 2 moles [if you write the equation: 2M(NO₃)₂ ----> 2MO + 4NO₂ + O₂ ], so we'll find the molar mass for one mole by dividing 328/2 and we get 164.

So now we know that the Mr of the metal + Mr of the nitrate => 164. We'll get the Mr of the nitrate which is 124, we subtract that from 164, we'll get Mr of the metal which is 40. Calcium which is Answer B has an Mr of 40!


For 2)
CH₃SH + 3O₂ ----> CO₂ + SO₂ + 2H₂O
1 mole of CH₃SH require 3 moles of 3O₂ to produce 1 mole of CO₂ + 1 mole of SO₂ which can be grouped into 2 moles of gases.
They're using 10cm³ of CH₃SH so by ration, only 30cm³ of O₂ will be required out of the given 60cm³, so the remaining O₂ would be 30cm³, in addition to that, since 2 moles of gases will be produced, and they are in the ratio 1:2, so 20cm³ of the gases will be produced, and hence 20+30=50cm³

For 3) It's pretty long, I'll write it down here later!