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2017 AQA A-Level Chemistry 7405/1 Unofficial Markscheme

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Reply 180

DarkEnergy

Original post by HHH7

Anyones else get +2 for Nitrogen in NO and -3 for NO3- ??

Pretty sure it's 5+ in NO3-

Reply 181

DarkEnergy

Original post by ugaaa5

Nein. Mg(OH)2 is sparingly soluble. The PH is around 9.

Original post by GenericEm

Does anyone remember the question asking how the pH would differ in magnesium hydroxide compared to calcium hydroxide?

I put that it would be lower as magnesium hydroxide is sparingly soluble and releases less OH- ions into the solution, i.e it ionises less in the water.

Any thoughts?

Didn't it say in the question that the Mg(OH)2 had fully dissolved though?

(edited 6 years ago)

Reply 182

Nik298

Original post by Competence

Optical isomerism is limited to molecules with one chiral carbon and 4 substituents? come on g

no optical isomerism is correct, you cant have cis trans with 3 ethandioate ligands :smile:

Reply 183

Deza28

Does anyone know what the concentrations of the ethanoic acid and ethanoate ions were in the 6 mark buffer?

Reply 184

username1869653

Original post by DarkEnergy

Didn't it say in the question that the Mg(OH)2 had fully dissolved though?

The wording was "until no more had dissolved"

Reply 185

DarkEnergy

Original post by ugaaa5

The wording was "until no more had dissolved"

Oh damn. Thanks

Reply 186

HHH7

Original post by DarkEnergy

Pretty sure it's 5+ in NO3-

Ah sh*t didn't know what i was thinking

Reply 187

Nik298

Original post by TianaEsther

This is a silly question, but how were we supposed to work out the mass of 49Ti using Avogadro's constant?

i got it wrong i think but : (1/ avogardos constant)x49 would give you the mass of a single ion

Reply 188

DarkEnergy

Original post by Nik298

i got it wrong i think but : (1/ avogardos constant)x49 would give you the mass of a single ion

Yeah exactly that's what it asked isn't it

Think I multiplied it by 1000 also because the m in n = m / Mr is in grams unless I've got that wrong

Reply 189

andzzzzzs

so did the ph of the with mg(oh)2 stay the same then

Reply 190

Nik298

Original post by DarkEnergy

Yeah exactly that's what it asked isn't it

Think I multiplied it by 1000 also because the m in n = m / Mr is in grams unless I've got that wrong

i didn't write that XD i had a dumb moment and just said the mass is 49, but thats for 1 mole of 49Ti ions :frown:

and not for a single ion (* this would produce a mass in g so you would have to then divide 1000 to get Kg)

(edited 6 years ago)

Reply 191

liasoe

Original post by DarkEnergy

Yeah exactly that's what it asked isn't it

Think I multiplied it by 1000 also because the m in n = m / Mr is in grams unless I've got that wrong

this is deffo right, my teacher went over this 5 mins before i went to the exam.. It's on ALL of the new papers.

Reply 192

rosemondtan

Original post by gommh

That's about what I got, units were KPa^-2 right? The mole fraction of methanol was enough to give it a partial pressure of a few (single digits I think) KPa, and the reactants were still accounting for way more of the overall pressure than the products. Glad I figured out an answer that someone else got, right or wrong :wink:

The question wanted what you saw with the addition of ammonia, which I thought meant in sequence, so blue precipitate of [Cu(H2O)4(OH)2] then a blue solution of [Cu(NH3)4(H2O)2]2+. Idk if they just wanted excess but I think that would've been a 1-2 mark question.

Anyone else really terrible at drawing the complex at the end? I got 2 of the ligands drawn before running out of space and just writing "octahedral" next to it lol

Yeah!! One of my friends got a couple hundred thousand or something for the kp and I was really confused but I'm pretty sure it's right, had about 30 minutes to double check my work so I'm pretty damn sure of my calculations (I damn well hope so at least)