Chemistry Marathon (HSC) (3 Viewers)

[Newbit]

A 20.0 mL sample of the solution was titrated with 0.100 mol L–1 hydrochloric acid.

When something like this is written, which solution is in the beurette and which is in the conical flask?

Need help with his question please.
A household cleaning agent contains a weak base with the formula NaX. 1.00 g of this
compound was dissolved in water to give 100.0 mL of solution. A 20.0 mL sample of the
solution was titrated with 0.100 mol L–1 hydrochloric acid, and required 24.4 mL of the acid for neutralisation.
What is the molar mass of the weak base?

 

[nightweaver066]

A 20.0 mL sample of the solution was titrated with 0.100 mol L–1 hydrochloric acid.

When something like this is written, which solution is in the beurette and which is in the conical flask?

Need help with his question please.
A household cleaning agent contains a weak base with the formula NaX. 1.00 g of this
compound was dissolved in water to give 100.0 mL of solution. A 20.0 mL sample of the
solution was titrated with 0.100 mol L–1 hydrochloric acid, and required 24.4 mL of the acid for neutralisation.
What is the molar mass of the weak base?

1. The hydrochloric acid is in the burette.

2. First finding moles of base neutralising the HCl,
n(HCl) = 0.0244 x 0.1 = 0.00244 moles
n(HCl):n(NaX) = 1:1
.'. n(NaX) = 0.00244 moles in a 20mL sample
.'. n(NaX) = 0.00244 x 5 = 0.0122 moles in a 100mL sample
n = m/MM
0.0122 = (1)/(MM)
MM = 1/0.0122 = 82.0g/mol (3sf)

 

[Xol_2]

I'm not sure if this question is somewhat universal, or "it just is." But, What makes up a Benzene ring? is it simply naturally occuring? Thought it was made from the carbon straight chains--

 

[dawooddaman]

I'm not sure if this question is somewhat universal, or "it just is." But, What makes up a Benzene ring? is it simply naturally occuring? Thought it was made from the carbon straight chains--

 

[Newbit]

I'm really struggling with titration questions

Citric acid, the predominant acid in lemon juice, is a triprotic acid. A student titrated 25.0 mL samples of lemon juice with 0.550 mol L–1 NaOH. The mean titration volume was 29.50 mL. The molar mass of citric acid is 192.12 g mol–1.
What was the concentration of citric acid in the lemon juice?

 

[Meshy590]

I'm really struggling with titration questions

Citric acid, the predominant acid in lemon juice, is a triprotic acid. A student titrated 25.0 mL samples of lemon juice with 0.550 mol L–1 NaOH. The mean titration volume was 29.50 mL. The molar mass of citric acid is 192.12 g mol–1.
What was the concentration of citric acid in the lemon juice?

1. write up equation. coefficient of NaOH will be 3 (because citric acid is triprotic)
2. find number of moles of NaOH. = conc x volume = 0.016225 moles
3. By stoichiometric ratios, number of moles of citric acid = one-third of the no of moles of NaOH. Therefore number of moles of citric = 0.005408333333... moles
4. n of moles = mass/molar mass. Therefore mass of citric = 1.039049 g
5. conc = mass of citric/mass of lemon juice x 100%= 4.156196%...=4.2% (2sf)

 

[nightweaver066]

I'm really struggling with titration questions

Citric acid, the predominant acid in lemon juice, is a triprotic acid. A student titrated 25.0 mL samples of lemon juice with 0.550 mol L–1 NaOH. The mean titration volume was 29.50 mL. The molar mass of citric acid is 192.12 g mol–1.
What was the concentration of citric acid in the lemon juice?

Let lemon juice be represented by the chemical formula H3X
3NaOH + H3X -> Na3X + 3H2O
n(NaOH) = 0.550 x 0.0295 = 0.016225 moles
n(NaOH) : n(H3X) = 3:1
.'. n(H3X) = 0.016225 /3 = 5.408 x 10^(-3) moles
m(H3X) = 5.408 x 10^(-3) x 192.12 = 1.039049 grams
.'. [H3X] = 1.039049/25 = 4.16% (g/mL) (3 sf)

 

[nucgaek]

Let lemon juice be represented by the chemical formula H3X
3NaOH + H3X -> Na3X + 3H2O
n(NaOH) = 0.550 x 0.0295 = 0.016225 moles
n(NaOH) : n(H3X) = 3:1
.'. n(H3X) = 0.016225 /3 = 5.408 x 10^(-3) moles
m(H3X) = 5.408 x 10^(-3) x 192.12 = 1.039049 grams
.'. [H3X] = 1.039049/25 = 4.16% (g/mL) (3 sf)

Just wondering, why not concentration in moles per litre?

 

[nightweaver066]

Just wondering, why not concentration in moles per litre?

I'm not too sure lol. I just used what the question gave.

 

[Newbit]

20 mL of 0.08 mol L–1 HCl is mixed with 30 mL of 0.05 mol L–1 NaOH.
What is the pH of the resultant solution?

The answer is 2.7, I keep getting 6.9, I don't know why..

 

[Lord Iron-Balls]

There are 1.6x10^-3 moles of HCl and 1.5x10^-3 moles of NaOH. Therefore NaOH is the limiting reagant and after neutralisation of all the NaOH, there is still 1x10^-4 moles of HCl. This is in 50mL of solution so the concentration of HCl is 2x10^-3. And now use the pH calculation thingo to get: 2.7

 

[tp1612]

industrial chem = shit load to memorise :L

so tru too many equations

 

[maths lover]

20 mL of 0.08 mol L–1 HCl is mixed with 30 mL of 0.05 mol L–1 NaOH.
What is the pH of the resultant solution?

The answer is 2.7, I keep getting 6.9, I don't know why..

0.02*0.08-0.03*0.05=0.0001
c=0.0001/0.05=0.002
ph=-log(0.002)=2.7

 

[Rawf]

To figure out TDS - if we find out the mass of non dissolved solids (filter paper) and then we figure out the mass of dissolved solids
EG: 2g of non-dissolved solids, 4g of dissolved solids and we have a 1L sample of water
is the % of TDS 4/(1000-2)x100 or is it just 4/(1000) x 100 ?

 

[someth1ng]

To figure out TDS - if we find out the mass of non dissolved solids (filter paper) and then we figure out the mass of dissolved solids
EG: 2g of non-dissolved solids, 4g of dissolved solids and we have a 1L sample of water
is the % of TDS 4/(1000-2)x100 or is it just 4/(1000) x 100 ?

I don't think you'd get penalised using either way but I would do the second method.

 

[freeeeee]

Ahh okay in one of my internals i did curve of best fit and i lost a mark though. I saw in successone that there was a graph with line of best fit, and it only had like 2 points on the line and 3-4 pretty off lolol. but it was for AAS though, should we be using a line of best fit for AAS curves?

line of best fit wont always work for AAS, this is because the line has to pass through (0,0)

 

[Rawf]

I don't think you'd get penalised using either way but I would do the second method.

I always go for the first method (minus the non dissolved solids), although the difference is only VERY slim (like non dissolved is usually like 0.1 or something). In successone, there are a few similar questions however they vary in the way they approach the question (some is the first method and some is the second) so I'm rather confused.
I don't understand they they give you info to figure out the non dissolved solids if you don't even need to use it when using the initial water sample mass

 

[someth1ng]

line of best fit wont always work for AAS, this is because the line has to pass through (0,0)

No, it doesn't have to pass through (0,0). You test a solution with concentration of 0 and you'll find that the absorbance is always above 0.

 

[freeeeee]

No, it doesn't have to pass through (0,0). You test a solution with concentration of 0 and you'll find that the absorbance is always above 0.

If concentration is 0, absorbence must be zero. I dont see how it can work any other way. My teacher penalised students in an assessment for not having AAS graph pass through (0,0)

 

[someth1ng]

If concentration is 0, absorbence must be zero. I dont see how it can work any other way. My teacher penalised students in an assessment for not having AAS graph pass through (0,0)

It doesn't have to go through zero because the light can get absorbed by other things in the air such as dust and that must be taken into account.