# Chemistry: Post your doubts here!

#### blootoomatoo

P×7000×10*-6=0.96/32×8.31×303

Ans:10.8 kPa

So it's C
Why do you multiply 7000 by 10*-6?

#### MUGABE King

OK so first you need to know the reactions of the period 3 oxides with water. AL2O3 does not react so optionA b are wrong. Next we react 2NaOH (which is a basic oxide and the product of reaction between oxide and H2O) with corresponding acidic oxide & balance the equations.
NOTE The Q states"neutralised by exactly 1mol....so D is correct as it balances perfectly with 2NaOH.
thank you

#### reinemiraa

The lattice energies(enthalpies) of rubidium fluoride(RbF) and caesium chloride(CsCl) are -760 KJ/mol and -650kJ/mol respectively. What is the lattice energy of caesium Fluoride(CsF) likely to be? (Rb-37;Cs-55) ANS B
A.-620kJ/mol
B.-720 kJ/mol
C.-800kJ/mol
D.-900kJ/mol
can any1 explain n is this an AS question?
hi , can i know you get B ?

#### reinemiraa

The lattice energies(enthalpies) of rubidium fluoride(RbF) and caesium chloride(CsCl) are -760 KJ/mol and -650kJ/mol respectively. What is the lattice energy of caesium Fluoride(CsF) likely to be? (Rb-37;Cs-55) ANS B
A.-620kJ/mol
B.-720 kJ/mol
C.-800kJ/mol
D.-900kJ/mol
can any1 explain n is this an AS question?
do you know the explanation?

#### reinemiraa

The lattice energies(enthalpies) of rubidium fluoride(RbF) and caesium chloride(CsCl) are -760 KJ/mol and -650kJ/mol respectively. What is the lattice energy of caesium Fluoride(CsF) likely to be? (Rb-37;Cs-55) ANS B
A.-620kJ/mol
B.-720 kJ/mol
C.-800kJ/mol
D.-900kJ/mol
can any1 explain how the answer is B

#### astro04

does anyone know what is the corresponding letters options for questions like this

When organic refuse decomposes in water carboxylic acids are formed. The water becomes acidic and aquatic life is destroyed.
Which additives are suitable to remove this acid pollution?
1 calcium carbonate
2 calcium hydroxide
3 potassium nitrate

The answer is B. Which options is it referring to?

• Mustfm

#### crack the code

does anyone know what is the corresponding letters options for questions like this

When organic refuse decomposes in water carboxylic acids are formed. The water becomes acidic and aquatic life is destroyed.
Which additives are suitable to remove this acid pollution?
1 calcium carbonate
2 calcium hydroxide
3 potassium nitrate

The answer is B. Which options is it referring to?
option 1 and 2

#### Samand

An element X and compound YZ react separately with acid as shown.
X(s) + 2H+(aq) → X2+(aq) + H2(g)
YZ(s) + 2H+(aq) → Y2+(aq) + H2Z(g)
When 1.0 g of either X or YZ is reacted with an excess of acid, the total volume of gas formed is the same.
Which statements are correct?
1. Ar(X) = Mr(YZ)
2. X and Y are metals.
3. X and Y must both be in the same group of the Periodic Table.

Why is the answer "1 and 2 are correct"?

#### coolhudhayfahfaiz

Hi guys, I have a question regarding Fe+2 and KMnO4 titrations. So you do know that before carrying out the titration excess dilute sulfuric acid must be added to the Fe+2 solution to allow redox rxn to occur. But doesn't the addition of excess dilute sulfuric acid(mainly water) dilute the Fe+2 solution in the conical flask?

#### coolhudhayfahfaiz

Re: All Chemistry help here!! Stuck somewhere? Ask here!

A 20.0mL sample of 0.200M HBr solution is titrated with 0.200M NaOH solution. Calculate the pH of the
solution after the following volumes of base have been added:
15 mL
ANYBODY ????

HBr + NaOH -----> NaBr + H20

n=cv=0.02 x 0.2 = 4 x10^-3 moles of HBr

n of NaOH = cv= 0.015 x 0.2= 3 x 10^-3 moles.

1x10^-3 moles of HBr left. Assuming HBr is a strong acid, [H+] = 1x10^-3/0.02+0.015=0.0286 mol dm^-3

ph=-log[H+]=1.54

#### crack the code

Hi guys, I have a question regarding Fe+2 and KMnO4 titrations. So you do know that before carrying out the titration excess dilute sulfuric acid must be added to the Fe+2 solution to allow redox rxn to occur. But doesn't the addition of excess dilute sulfuric acid(mainly water) dilute the Fe+2 solution in the conical flask?
excess dilute acid is added to ensure that the reaction goes to completion only when all of fe2+ ions have been consumed by Mno4- ions and not due to decreased concentration of H+

Last edited:

#### garyGA

Suggest a test to determine if a MgO substance is mixture or compound. Thank you!

#### Ani876

X is the oxide of a Period 3 element. X reacts with water to give an acidic solution.
A solution is prepared by reacting 0.100g of X with excess water. This solution was neutralised
by exactly 25.0cm3
of 0.100moldm–3 sodium hydroxide solution.
What could be the identity of X?
A Al 2O3 B MgO C P4O10 D SO3

Need help with this

#### Mustfm

X is the oxide of a Period 3 element. X reacts with water to give an acidic solution.
A solution is prepared by reacting 0.100g of X with excess water. This solution was neutralised
by exactly 25.0cm3
of 0.100moldm–3 sodium hydroxide solution.
What could be the identity of X?
A Al 2O3 B MgO C P4O10 D SO3

Need help with this
Acidic oxide of period 3 means its either A, C, D
When so3 is dissolved in water it forms h2so4
H2so4 + 2naoh = na2so4 + 2h2o
note the mole ration of acid to base is 1: 2

Moles of naoh = concentration x volume
0.1 x 25/1000
= 2.5x10^-3
0.100g of x means that if its so3 then
mole = 0.100/16x3 +32
= 1.25x10^-3
which is exactly half
hence ans is D
hope it helps<3

• PlanetMaster

#### Ani876

Acidic oxide of period 3 means its either A, C, D
When so3 is dissolved in water it forms h2so4
H2so4 + 2naoh = na2so4 + 2h2o
note the mole ration of acid to base is 1: 2

Moles of naoh = concentration x volume
0.1 x 25/1000
= 2.5x10^-3
0.100g of x means that if its so3 then
mole = 0.100/16x3 +32
= 1.25x10^-3
which is exactly half
hence ans is D
hope it helps<3
Thank you soo much

• PlanetMaster