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OCR B F335 - Chemistry by Design - 13th June 2012

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Calculate the iodine number of DHA.
Mr: DHA, 328; Ar: I, 127
Give your answer to a suitable number of significant figures.

Reply 321

Bi0logical

Why is it that some molecules absorb UV light and some absorb in the visible?

Reply 322

sydney07

hey, just been flicking through my book and we don't need to learn the colours of the azo compounds do we? on pages 330-331? Because there's only three colours....

Reply 323

Princess Kawaii

Do we need to know about solubility Ksp? for the new spec/paper?

for example question 4(d) in the legacy paper Jan 10....

Reply 324

Princess Kawaii

Original post by Bi0logical

Calculate the iodine number of DHA.
Mr: DHA, 328; Ar: I, 127
Give your answer to a suitable number of significant figures.

I2: 254

254/328=0.77439
0.77439 x 100 = 77.439
= 77

Is that the way it goes? :smile:

Reply 325

Princess Kawaii

Help on Question 5(f) please ... June 10 paper

http://pdf.ocr.org.uk/download/pp_10_jun/ocr_57559_pp_10_jun_gce_f335.pdf?

:smile:

Reply 326

Bi0logical

Typical exam answer when asked if temperature is increased for an endothermic reaction:

(Forward) reaction is endothermic (1); 3
Equilibrium (position) moves to oppose change/ Increased temperature moves
equilibrium (position) in endothermic direction(1);
i.e. in direction consistent with stated endo/exo of forward reaction(1); (you can then go onto saying it's increased yield of product if asked)

Reply 327

SimpleGirl

Original post by Princess Kawaii

Help on Question 5(f) please ... June 10 paper

http://pdf.ocr.org.uk/download/pp_10_jun/ocr_57559_pp_10_jun_gce_f335.pdf?

:smile:

Use the formula for pH. You need to rearrange it to work out the value of [H⁺]. All the other values you need are given to you.

Check the spoiler for the full method. :smile:

Spoiler

(edited 11 years ago)

Reply 328

Bi0logical

Original post by Princess Kawaii

I2: 254

254/328=0.77439
0.77439 x 100 = 77.439
= 77

Is that the way it goes? :smile:

I'm just marking the paper, once I come to this questions I'll tell you if you did it right though, thanks

Reply 329

Bi0logical

Describing why ionic compounds e.g. NaCl have a high melting point and can conduct electricity:

ionic (1); strong electrostatic forces/bonds (lead to high melting point) (1);
(dissolves because) ions are hydrated/ ions form (ion-dipole) bonds/ ions attract
water moleules (1);
conducts because (free) ions can move (1)

Reply 330

Bi0logical

Original post by Princess Kawaii

I2: 254

254/328=0.77439
0.77439 x 100 = 77.439
= 77

Is that the way it goes? :smile:

Mark scheme:

6 x 254 g of iodine (per 328 g DHA)*
Iodine no. = 1524 *(ecf) x 100/328 = 465 **

I dont get this, can someone take a look it's June 2006 I think you have to look at the structural formula too.

Reply 331

Bi0logical

Original post by SimpleGirl

Use the formula for pH. You need to rearrange it to work out the value of [H⁺]. All the other values you need are given to you.

Check the spoiler for the full method. :smile:

Spoiler

Excellent explanation, do you have method like these for each calculation they can ask us? It would be VERY useful

Reply 332

Bi0logical

Form B is coloured while form A absorbs in the UV. Heres why:

Form B absorbs (certain frequencies of) visible light;
(when) electrons excited (to higher energy-level);
Form B is more delocalised/larger chromophore/ larger conjugated system/ more
conjugated (NOT extra double bond) (ora for Form A)
because of alternating single and double bonds joining (benzene) rings (ora for
Form A)
Form B needs less energy to excite electrons (ora for Form A)/visible light has
lower energy than u.v.;
energy level difference measures frequency/wavelength absorbed/
(∆)E = hv;
max 2 if emission of light (rather than transmission) is implied

Reply 333

Bi0logical

Guys, how does a buffer solution actually work? is this right
Addition of H+ moves the equilibrium to the left, this removes H+ and maintains the pH
The addition of salt, also makes the equilibrium shift to the left by reacting A- with H+ to form acid (maintaining pH?)

Reply 334

Princess Kawaii

Original post by Bi0logical

Oh okay...I did it like that for another question before and got it right.
Yeh probably have to look at the formula to get the "6" part of it.

I don't have that paper...can u put a link or attach it please :smile:

Reply 335

Princess Kawaii

Original post by Bi0logical

- When small amounts of acid is added the equilibrium shifts to the left to oppose the change/ remove the H+.
- When small amounts of a alkali is added the equilibrium shiftsto the right to oppose the change
- pH remains constant because there is a large amount of HA and A-

Reply 336

SimpleGirl

Original post by Bi0logical

Excellent explanation, do you have method like these for each calculation they can ask us? It would be VERY useful

Glad it helped :smile:

What other calculation questions do you need help on? I need an example to work through.

Reply 337

Jasmine_777

when does the rate constant increase?
I know it increases when the yield of a reaction increases.
and does it always increase with temp, increased pressure, conc ?