TrueTears question thread

[TrueTears]

Ok, i think my volumetric one is wrong here's my new working, i don't see anything wrong but the results do not match up with my gravimetric results ( and they should)

We then did another prac, but this time we used volumetric analysis(back titration) to do the same thing.



Aim: Find the mass of ammonium sulfate in a brand of lawn fertiliser

Mass of lawn fertiliser: 1.301g

Here are the steps to the prac

1. Using a small funnel, carefully transfer the sample of the fertiliser to the volumetric flask (250mL). Add water until it reaches the calibration line (250mL).

2. Using a 20.00mL pipette, dispense aliquots of the fertiliser solution into each of the 3 conical flasks.

3. Thoroughly wash the pipette and rinse it with a small volume of standard sodium hydroxide solution. Place a 20.00mL aliquot of NaOH solution in each of the flasks containing fertiliser solution. The NaOH solution has a concentration of 0.1M

4. Rinse and fill a burette with standard hydrochloric acid, concentration is 0.09933M

5. add 4-6 drops of methyl red indicator to each conical flask containing the fertiliser solution and titrate.

The average titre i got was 16.335mL

Here is my working out:

Original n(NaOH) = = 0.002 mol



= 0.00162454 mol

= 0.00162454 mol

= 0.002 - 0.00162454 = 0.000375458mol

The ionic reaction between ammonium sulfate and NaOH is as follows:













so mol



But the mass is meant to be approximately between 0.600 - 0.700 g

and this result doesn't match with my gravimetric prac result which yields a mass of 0.687g of ammonium sulfate

can anyone see what's wrong?

Z.Z.Z just checked it like 4 times, still can't see what's wrong !!!ZZ>Z>Z>Z

[TrueTears]

This was my graph for part 2 (bad paint)



so for question 3, how do you know the x value of the x intercept, if you 'extrapolate' backwards...?

[shinny]

Ahh I remember doing this last year...well from what I can see, you'll just have to estimate (with the aid of a ruler of course) roughly what 'm minus' value it'd be by drawing the line backwards and measuring how far from the centre the x int is. I don't think they're expecting anything ridiculously accurate in Chem...

[TrueTears]

thanks shinny i got it XD

[TrueTears]

Polypropene is made by polymerization of propene, . One sample of this polymer is found to have a molar mass of approximately . The number of carbon atoms in one molecule of this polymer would be closet to:

A 2500
B 2900
C 7500
D 8750

EDIT: answer is C

[Mao]

the mass of the monomer is 42

since addition polymerisation does not produce any waste product, 105000/42 = 2500 monomers

there are 3 carbon in each monomer, hence 7500 carbon in the polymer

[TrueTears]

YAY mao's confuse ray on me stopped

thanks

[Over9000]

the mass of the monomer is 42

since addition polymerisation does not produce any waste product, 105000/42 = 2500 monomers

there are 3 carbon in each monomer, hence 7500 carbon in the polymer

so pr0 my mind hurts

[TrueTears]

A sample of nitogen() has a mass of 10g and a volume of 8L. Which of the following values can be calculated for the nitrogen sample from these data.

1. moles
2. % w/v
3. density

A 1 only
B 1 and 2 only
C 1 and 3 only
D 1,2 and 3

[Mao]

, so number of moles can be calculated

mass and volume is known, hence concentration in % w/v can also be calculated (the unit is g/100mL)

density is another measurement for mass per unit volume, which can be calculated from the % w/v

all three quantities can be calculated, hence D.

[TrueTears]

That's what I picked as well, but answer says B -_-

Wrong answers

[TrueTears]

Give the systematic name for this:

The book says the longest chain is 6 carbon, but I can only find 5...?

Also how can the 3rd carbon atom counting from left to right on the top row be , that would mean that Carbon atom has 5 bonds? Is the question wrong or am I just misinterpreting it?

[Mao]

I believe it is a typo

Code: [Select]

CH3-CH-CH2-CH--CH3
    |      |
    CH3    CH2-CH3


[TrueTears]

^Thanks mao

Also this new question:

Glycine, , is one of the amino acids that forms proteins. Protein that is not required in the body is broken down in the liver. Unwanted nitrogen is converted into urea. and eliminated in the urine. The maximum mass of urea (relative molecular mass = 60) that could be eliminated as a result of the breakdown of 1.00 g of glycine (relative molecular mass = 75) is, in grams?
A. 0.40
B. 0.80
C. 1.00
D. 1.60

Thanks in advance.

[chem-nerd]

n(CO(NH₂)₂) = 1/2 n(H₂NCH₂COOH)

thus, A