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Author Topic: VCE Chemistry Question Thread  (Read 2313829 times)  Share 

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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #210 on: January 25, 2014, 04:49:31 pm »
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Quick Q about pH curves

In the image i've attached, the pH curve on the right is a titration between a strong acid and strong base. Why is phenophthalein used as the indicator? I thought it would be bromothymol blue? There's still a sharp end point between 6.0 - 7.6

To determine which indicator is best to use, look at the centre of the almost vertical drop (i.e the end point) for the equivalence point pH. Using that value, determine which is the BEST indicator.

Jason12

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Re: VCE Chemistry Question Thread
« Reply #211 on: January 25, 2014, 05:00:43 pm »
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In a chemical reaction how can you tell which species is acting an acid?

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Yacoubb

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Re: VCE Chemistry Question Thread
« Reply #212 on: January 25, 2014, 05:02:17 pm »
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In a chemical reaction how can you tell which species is acting an acid?

The species that donates a proton/s (i.e. H+ ion).

Bluegirl

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Re: VCE Chemistry Question Thread
« Reply #213 on: January 25, 2014, 05:06:14 pm »
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how do i write and equation for the oxidation of nitrogen(II) oxide in air?

Jason12

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Re: VCE Chemistry Question Thread
« Reply #214 on: January 25, 2014, 05:07:07 pm »
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The species that donates a proton/s (i.e. H+ ion).

so if there is NH4+ on the reactants side and the product was NH3 does this mean it donated the H+?
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #215 on: January 25, 2014, 05:22:07 pm »
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so if there is NH4+ on the reactants side and the product was NH3 does this mean it donated the H+?

Yep! NH4+ would be the acid and NH3 would be it's conjugate base.

The NH4+ would've transferred or "donated" it's proton (H+ ion), causing it to become NH3
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Re: VCE Chemistry Question Thread
« Reply #216 on: January 25, 2014, 05:25:41 pm »
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how do i write and equation for the oxidation of nitrogen(II) oxide in air?
2NO(g) + O2(g) ----> 2NO2(g)

so if there is NH4+ on the reactants side and the product was NH3 does this mean it donated the H+?
Yes since NH4+ lost a H+ it means it donated that H+ to whatever else was on the reactant side to become NH3. So NH4+ acted as an acid.
 
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Re: VCE Chemistry Question Thread
« Reply #217 on: January 25, 2014, 05:26:30 pm »
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T-Infinite

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Re: VCE Chemistry Question Thread
« Reply #218 on: January 25, 2014, 07:20:50 pm »
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Please help me with this question.

Assign oxidation numbers to each element in this equation, and hence identify the oxidant and reductant

Fe2O3 (s) + 3CO (g) -----> 2Fe (s) + 3CO2 (g)

I get so confused with the whole oxidant/reductant/oxidation/reduction thing. @___@
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tinkerbell

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Re: VCE Chemistry Question Thread
« Reply #219 on: January 25, 2014, 07:35:51 pm »
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Please help me with this question.

Assign oxidation numbers to each element in this equation, and hence identify the oxidant and reductant

Fe2O3 (s) + 3CO (g) -----> 2Fe (s) + 3CO2 (g)

I get so confused with the whole oxidant/reductant/oxidation/reduction thing. @___@

To assign oxidation numbers, you have to follow some rules, one of them being that a free element such as iron, which is a product in this reaction, has an oxidation number of 0.

Another important rule is that a compound has a net oxidation number of 0-so adding up the oxidation numbers of the individual elements making up the compound. So,  Fe203 is a compound and it must have a net oxidation charge of 0. Valence of oxygen is 2 and we know oxygen is electronegative (has an affinity for electrons), so the oxidation number for oxygen is -2 times 3 (because there are 3 oxygen atoms and we are accounting for all of them).

So, this means that the Fe2 component must have a positive charge of +6. This charge is distributed equally between the 2 Fe atoms.

Let's look at CO-oxygen -2, Carbon must have +2 oxidation number.

CO2-oxygen negative 4, so carbon positive 4.

Now that we have assigned our oxidation numbers, our next step is to work out what is the oxidising agent and what is the reducing agent.

Oxidising agent-what oxidises something and itself gets reduced. To be reduced the oxidation number needs to decrease. We see this with Fe203, which is a reactant and leads to the free element Fe. So, this is a oxidising agent/reductant.

CO (carbon monoxide) is the reducing agent because it gets oxidised-the oxidation number on the carbon atom increases.

Increase in oxidation number means-become oxidised and therefore reducing agent. reducing agent reduces Fe2O3 (because you get free iron).

Hope this makes sense,

Good luck.

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Re: VCE Chemistry Question Thread
« Reply #220 on: January 25, 2014, 07:47:13 pm »
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To assign oxidation numbers, you have to follow some rules, one of them being that a free element such as iron, which is a product in this reaction, has an oxidation number of 0.

Another important rule is that a compound has a net oxidation number of 0-so adding up the oxidation numbers of the individual elements making up the compound. So,  Fe203 is a compound and it must have a net oxidation charge of 0. Valence of oxygen is 2 and we know oxygen is electronegative (has an affinity for electrons), so the oxidation number for oxygen is -2 times 3 (because there are 3 oxygen atoms and we are accounting for all of them).

So, this means that the Fe2 component must have a positive charge of +6. This charge is distributed equally between the 2 Fe atoms.

Let's look at CO-oxygen -2, Carbon must have +2 oxidation number.

CO2-oxygen negative 4, so carbon positive 4.

Now that we have assigned our oxidation numbers, our next step is to work out what is the oxidising agent and what is the reducing agent.

Oxidising agent-what oxidises something and itself gets reduced. To be reduced the oxidation number needs to decrease. We see this with Fe203, which is a reactant and leads to the free element Fe. So, this is a oxidising agent/reductant.

CO (carbon monoxide) is the reducing agent because it gets oxidised-the oxidation number on the carbon atom increases.

Increase in oxidation number means-become oxidised and therefore reducing agent. reducing agent reduces Fe2O3 (because you get free iron).

Hope this makes sense,

Good luck.
Thank you! Your step-by-step explanation really helps :) !
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PsychoT

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Re: VCE Chemistry Question Thread
« Reply #221 on: January 25, 2014, 10:20:31 pm »
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Can anyone help me out with this?

The percentage of iron in steel wool was analysed by dissolving 0.1674g in 50ml  of sulfuric acid to produce Fe2+. These ions were then titrated against 0.0213M Mn04- solution, producing Fe3+ and Mn2+ ions. The volume of permanganate solution required was 26.46 ml. Calculate the percentage of iron in the steel wool.
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Re: VCE Chemistry Question Thread
« Reply #222 on: January 25, 2014, 11:19:25 pm »
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Lets write down some equations first regarding this reaction.
Oxidation: ---> + (multiply by 5 and add oxidation and reduction equation together)
Reduction:
Overall:

So now lets start some calculations : x

x



x

Therefore :   % Fe = x = 94.1 % (3 sig fig)

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« Last Edit: January 25, 2014, 11:25:30 pm by LOLs99 »
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Jason12

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Re: VCE Chemistry Question Thread
« Reply #223 on: January 25, 2014, 11:34:10 pm »
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what is the difference between the relative molecular mass, Mr of water and the molar mass of water?
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brightsky

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Re: VCE Chemistry Question Thread
« Reply #224 on: January 25, 2014, 11:53:06 pm »
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not much, except relative molecular mass has no units but molar mass has units g/mol.
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