Homework questions thread

[Bhootnike]

Saturation/unsaturation relates to the bonding between the carbons only, hence the C=O does not count

okay, but shouldnt DBE formula still give number of double bonds ?
because ethanoic acid: c2h402, has DBE=(2(2+1)-4 )/2 => 2/2 => 1 , meaning 1 double bond in it! ?

im guessing that for fatty acids specifically, the DBE will only give the number  of double bonds b/w carbon then ?

[destain]

ah okay 2HBr....
ad what does reacting under reflux mean?
does that mean it's a 1:1 ratio?

^ and the conversion of chloroethane to ethylamine, isn't the only reagent NH3, and a catalyst of UV light?
Why does the answers say, reaget of NH3 followed by NaOH

[Comatose]

hey guys can someone explain how to do this (from stav 2009) i looked at the solutions and i still dont understand it
An antacid is known to contain 400 mg of magnesium hydroxide, Mg(OH)₂ (M = 58.3 g mol), per 10.0 mL. The number of mole of hydroxide ions in 1.0 L of the antacid would be closest to
A. 1.4 X 10-3
B. 1.4 X 10-2
C. 1.4 X 10-1
D. 1.4
answer is D thanks

[Lasercookie]

ad what does reacting under reflux mean?
does that mean it's a 1:1 ratio?

Someone might want to double check this, but reflux is a technique where you collect any vapours and condense it back so you don't lose that amount for your measurements.

You don't need to know it, I think I remember the question you 'may' be talking about, to answer that question you just needed to apply the knowledge you already know.

Usually if there are unfamiliar words like that in VCAA exam questions, they stick in a short sentence explaining what it does (comes under one of the key skills dot points in the study design, something like 'applying knowledge in unfamiliar situations').

[destain]

But question says something like an ester reacts with NaOH, how do i know it's a 1:1 reaction :S...
and in organic reaction pathways, from like ethene to ethanol
or ethanol + ethanoic acid, do i always need to say in H+ acidic solution? is that always a reagent?

[Bhootnike]

hey guys can someone explain how to do this (from stav 2009) i looked at the solutions and i still dont understand it
An antacid is known to contain 400 mg of magnesium hydroxide, Mg(OH)₂ (M = 58.3 g mol), per 10.0 mL. The number of mole of hydroxide ions in 1.0 L of the antacid would be closest to
A. 1.4 X 10-3
B. 1.4 X 10-2
C. 1.4 X 10-1
D. 1.4
answer is D thanks

400mg = 0.4g
10ml goes 100 times into 1L
question asks to find mole of OH- ions, so molar mass = 34, since its (OH)2
so its just: 0.4/34 =0.0118ish  mol in 10mL
0.0118ish x 100 in 1 L
1.18ish in 1L
so D is closest to that.
hope im right!

bump! :

for those who have thushans book - pg 242, q1c,
Why is it that the m/z value of the largest molecular ion will increase throughout a analysis, given that we have a mixture of alkanes which are analysed through a coupling technique, where the sample is inserted into a gas chromatograph, and then travels into a mass spectrometer.

is it right that when in GC, the mixture has all the alkanes together, which are soluble in one another, but then separated due to adsorption onto s'phase. So when the largest alkane does reach the mass spec, itll be by itself?

i dont get how 'm/z of molecular ion will increase over time'

^ this and,

Saturation/unsaturation relates to the bonding between the carbons only, hence the C=O does not count

okay, but shouldnt DBE formula still give number of double bonds ?
because ethanoic acid: c2h402, has DBE=(2(2+1)-4 )/2 => 2/2 => 1 , meaning 1 double bond in it! ?

im guessing that for fatty acids specifically, the DBE will only give the number  of double bonds b/w carbon then ?

[Lasercookie]

Saturation/unsaturation relates to the bonding between the carbons only, hence the C=O does not count

okay, but shouldnt DBE formula still give number of double bonds ?
because ethanoic acid: c2h402, has DBE=(2(2+1)-4 )/2 => 2/2 => 1 , meaning 1 double bond in it! ?

im guessing that for fatty acids specifically, the DBE will only give the number  of double bonds b/w carbon then ?

The DBE formula will give the number of double bonds including the C=O in COOH group (hence why you get the value of 1 for Ethanoic Acid). 

[ggxoxo]

The level of carbon dioxide in the air in a spacecraft can be controlled by passing the air through canisters
containing lithium hydroxide, LiOH.
In a laboratory trial, the air in a 5.00 L container at 1.10 × 102 kPa and 25.0°C was passed through a canister of
LiOH. The pressure of the air in the container decreased to 1.00 × 102 kPa, measured at 25.0°C.

For ths question, how did they derive p(CO2) (i.e. why did VCAA subtract the 2 pressures given?)?

[Mao]

The level of carbon dioxide in the air in a spacecraft can be controlled by passing the air through canisters
containing lithium hydroxide, LiOH.
In a laboratory trial, the air in a 5.00 L container at 1.10 × 102 kPa and 25.0°C was passed through a canister of
LiOH. The pressure of the air in the container decreased to 1.00 × 102 kPa, measured at 25.0°C.

For ths question, how did they derive p(CO2) (i.e. why did VCAA subtract the 2 pressures given?)?

Air contains O2, N2, trace amounts of CO2, Ar, etc.

If it is given that LiOH scrubs CO2, then the difference in pressure must be the partial pressure of CO2 removed from air.

[Mao]

is it right that when in GC, the mixture has all the alkanes together, which are soluble in one another, but then separated due to adsorption onto s'phase. So when the largest alkane does reach the mass spec, itll be by itself?

More importantly, the alkanes are soluble in the mobile phase, this is why we have a mobile phase.

Longer alkanes have stronger adsorption onto the stationary phase, thus they will be retained for longer. Small alkanes have a low retention time, larger alkanes have higher retention times, thus the molecular ion peak appears to increase over time. This increase is not gradual nor continuous, it's going on the principle that larger molecule --> greater retention --> longer time.

Also, please don't repeatedly quote your own questions. If you think they're getting 'buried' in a fast-moving thread, start a new thread.

[Mao]

Aldehydes are primary alcohols that have been partially oxidised. If you further oxidise them they will be converted to a carboxylic acid, so A is correct.

AFAIK this isn't in the VCAA Chemistry course.

I don't think there's any reason it shouldn't be on the course. If you look at the oxidation number of C in alcohol, aldehyde and carboxylic acids, it is reasonably clear that those are different steps in oxidation numbers.

[Mao]

ad what does reacting under reflux mean?
does that mean it's a 1:1 ratio?

Someone might want to double check this, but reflux is a technique where you collect any vapours and condense it back so you don't lose that amount for your measurements.

You don't need to know it, I think I remember the question you 'may' be talking about, to answer that question you just needed to apply the knowledge you already know.

Usually if there are unfamiliar words like that in VCAA exam questions, they stick in a short sentence explaining what it does (comes under one of the key skills dot points in the study design, something like 'applying knowledge in unfamiliar situations').

Don't worry about reaction under reflux. That's a technique we use to keep the reaction mixture at a certain temperature (at the boiling point of the solvent). Essentially, the solvent is boiling, and the vapour rises to the condenser and comes back down as liquid, so the whole mixture is kept at the boiling point. This is necessary as some reactions take a long time to happen (several hours, or even days). There would be plenty of videos on youtube.

[Mao]

But question says something like an ester reacts with NaOH, how do i know it's a 1:1 reaction :S...
and in organic reaction pathways, from like ethene to ethanol
or ethanol + ethanoic acid, do i always need to say in H+ acidic solution? is that always a reagent?

Try balancing an equation for the hydrolysis of an ester, and convince yourself that only 1 water is required. Same thing applies for NaOH, except you form the sodium salt of the acid (sodium carboxylate) instead of the carboxylic acid.

For the esterification, conc acid is the catalyst.

[Mao]

ah okay 2HBr....
ad what does reacting under reflux mean?
does that mean it's a 1:1 ratio?

^ and the conversion of chloroethane to ethylamine, isn't the only reagent NH3, and a catalyst of UV light?
Why does the answers say, reaget of NH3 followed by NaOH

I don't think UV light is necessary here. It is needed for chlorination of ethane, but not in this case.

For this reaction to proceed, the environment needs to be reasonably basic. The substitution of amine will produce HCl as a by-product, so this must be neutralised.

[charmanderp]

Aldehydes are primary alcohols that have been partially oxidised. If you further oxidise them they will be converted to a carboxylic acid, so A is correct.

AFAIK this isn't in the VCAA Chemistry course.

I don't think there's any reason it shouldn't be on the course. If you look at the oxidation number of C in alcohol, aldehyde and carboxylic acids, it is reasonably clear that those are different steps in oxidation numbers.

That's true, but wouldn't it be a bit unfair to expect students to recognise aldehydes? I'm pretty sure the intention of that question was to convince them that none of A, B or C were primary alcohols or alcohols at all, pushing them to choose D.