[bear_007]

1/ In the reaction below, 0.3 moles of sodium were added to 0.2 moles of bromine.

2Na + Br₂ -> 2NaBr

Which was the limiting reactant?

Correct answer: Sodium

My answer: Bromine because 0.2 moles of Bromine react with 0.2 moles of Sodium. Sodium is in the excess with 0.3 moles, so Bromine is the limiting reactant.


2/During a titration, 22.5cm³ of 0.20mol/dm³ hydrochloric acid neturalised 25.00cm³ of sodium hydroxide solution.

The equation for the reaction is shown below:

NaOH_(aq) + HCl_(aq) -> NaCl_(aq) + H₂O_(l)

a) How many moles are present in 22.5 cm³ of 0.20mol/dm³ hydrochloric acid?

My answer is correct
: number of moles = concentration x volume
0.2 x 0.0225dm³= 0.0045 moles of HCl

b) How many moles of sodium hydroxide were present in the 25.00cm³ of sodium hydroxide. Explain your answer.

My answer is correct: 0.0045 moles of NaOH because same amount required to neutralise acid. 1:1 ratio of moles of NaOH to HCl.

c) Calculate the concentration of the sodium hydroxide solution in mol/dm³

Correct answer: (0.0045 /1000) / 25 = 0.18 mol/dm³

^
This doesn't make sense to me.
My answer: 0.0045/ (25/1000) = 1.8 mol/dm³

Reason because the 25 is in cm³ so to convert into dm³, I would divide it by a thousand because my answer needs to be in dm³. Please resolve my misunderstanding.

3/Calculate the concentration of the sodium hydroxide solution in g/dm³

Correct answer= 0.18 x 40 = 7.2g/dm³

My answer: Mass= Moles x Mr

0.0045 x  (23+1+16)
= 0.18g/dm³

But my answer is wrong. Why did the use the concentration when the formula uses moles to work out the mass?


Please advise.

[AWK]

1.
write the reaction in another way
2Na + 2Br
Do you see your error?

2c
This makes sense, but you cannot do calculations

3.
0.0045 moles is in your sample. You should calculate content of NaOH in 1 dm³.

[chenbeier]

1.
write the reaction in another way
2Na + 2Br
Do you see your error?

why change in this way？

2 Na + Br₂ => 2NaBr


The ratio is 2:1

If 0.3 mol is Na used then we need only 0.15 mol bromine, what means sodium is limited compound
Calculatiom in opposit means if 0.2 mol bromine is used then it reqires 0.4 mol Na , but only 0.3 is available so Na still limited agemt.

[AWK]

Bear_007 understood reaction as 1:1

My answer: Bromine because 0.2 moles of Bromine react with 0.2 moles of Sodium

[chenbeier]

This is of course a mistake.His explanation is wrong.

[bear_007]

1.
write the reaction in another way
2Na + 2Br
Do you see your error?

why change in this way？

2 Na + Br₂ => 2NaBr


The ratio is 2:1

If 0.3 mol is Na used then we need only 0.15 mol bromine, what means sodium is limited compound
Calculatiom in opposit means if 0.2 mol bromine is used then it reqires 0.4 mol Na , but only 0.3 is available so Na still limited agemt.

Understood blue's explanation but not red because bromine is the limiting reactant because there's less of it. There's  plenty of sodium.

[bear_007]

1.
write the reaction in another way
2Na + 2Br
Do you see your error?

2c
This makes sense, but you cannot do calculations

3.
0.0045 moles is in your sample. You should calculate content of NaOH in 1 dm³.

Understood 3. Thanks but the first two you quoted, I don't understand.

[AWK]

1. I tried to show you that you can check your understanding of changing bromine molecules to atoms.
This is the way used by chemist at the beginning of the XIX century. Then all reactions have 1:1 stoichiometry

3.
 0.0045/ (25/1000) = 1000 x 0.0045/25 = ?

[chenbeier]

@ Bear..007 Again if you have 0,3 mol Na reacting with bromine you would consume 0,15 mol of it. But 0,2 mol is available, so bromine is excess and sodium limited.