chemistry 1/2 questions thread

[MNM101]

It's a practical experiment where I had to dissolve 0.5g of KMnO4 in 500mL water in a 500mL flask. The question is then asking what the volume of the solution is in mL and what the mass of KMn04 is( dissolved in the solution)

[jgoudie]

Hmmm, seems like a pretty strange question as you have already answered it. The volume is 500ml, the mass is 0.5g.

What would make more sense is if they wanted you to find the concentration.  This can be found in gram/ml or mol/L.  For each of these just do what the units tell you.

grams/ml -> divide mass by volume (ml)
mol/L -> divide mol by volume (L)

It's a practical experiment where I had to dissolve 0.5g of KMnO4 in 500mL water in a 500mL flask. The question is then asking what the volume of the solution is in mL and what the mass of KMn04 is( dissolved in the solution)

[MNM101]

Hi, I'm trying to write the observations&discussion for a supersaturation experiment. The substance we tested was sodium thiosulfate. I'm just having a hard time trying to write down the discussion section, I always have problems with this section as I barely write any prac reports, so can anyone help, thanks in advance.

[jgoudie]

Here is a video on super saturation.  Most pracs will pretty much deal how supersaturation is formed, what must happen to trigger crystallisation and what type of reaction (exothermic) it is.  You might also need to explain where it is used in real life, i.e. heat packs and what not.

Hi, I'm trying to write the observations&discussion for a supersaturation experiment. The substance we tested was sodium thiosulfate. I'm just having a hard time trying to write down the discussion section, I always have problems with this section as I barely write any prac reports, so can anyone help, thanks in advance.

[MNM101]

Here is a video on super saturation.  Most pracs will pretty much deal how supersaturation is formed, what must happen to trigger crystallisation and what type of reaction (exothermic) it is.  You might also need to explain where it is used in real life, i.e. heat packs and what not.

Thank you that was so helpful!

[MNM101]

For this question it's asking to calculate the volume of solution in mL, mass if the substance dissolved in g/mL and the concentration in mol/L of 0.5g of KMnO4 which has been dissolved in a 500mL flask and then had 450mL tipped out of it, and then had 450mL of water added to that remaining solution (50mL) , it a dilution process but the question is asking for the concentration so I'm not sure what to do. Do I use c1v1=c2v2 or just c=n/V or c=m/V? Also can you show the working out? Thanks a lot

[jgoudie]

Work through it methodical.

Find the concentration in the first 500ml. (n=m/Mr, then c=n/V)

The next step is the dilution.
so C1V1 = C2V2

C1 = concentration found in part a
V1 = volume of this, (your 50ml you have left)
V2 = the volume you are diluting to (450ml + 50ml = 500ml)

For this question it's asking to calculate the volume of solution in mL, mass if the substance dissolved in g/mL and the concentration in mol/L of 0.5g of KMnO4 which has been dissolved in a 500mL flask and then had 450mL tipped out of it, and then had 450mL of water added to that remaining solution (50mL) , it a dilution process but the question is asking for the concentration so I'm not sure what to do. Do I use c1v1=c2v2 or just c=n/V or c=m/V? Also can you show the working out? Thanks a lot

[dankfrank420]

Remember that a hydrogen atom is treated as the same thing as a proton. The NH3 was balanced and with the addition of a proton (H+ atom) it has a charge of 1+.

[RazzMeTazz]

Are the products of an acid-base reaction always salt and water?

Because  when you have lets say for example, PO43- as the Bronsted-Lowry base and HNO3 as Bronsted-Lowry acid, you form NO3- and HPO42-.

And hence the products of this acid-base reaction are not a salt and water?

[MNM101]

In an experiment I performed, 20 drops of HCl was added with a drop of methyl indicator, the pH was 8 as it was a light blue, but what does the pH value tell u about the extent of hydrolysis of HCl and is it a strong or weak acid?

[RazzMeTazz]

Do allotropes of an element have different chemical and physical properties?

[zsteve]

@RazzMeTazz: They definitely do: take, for example vs
is colorless, vital for life and combustion.
is volatile, bluish, and kills everything it sees
Also, take graphite and diamond, allotropes of carbon

@MNM101:
HCl can be assumed to ionize virtually completely in water. The fact that your pH is reading 8 is weird - no matter what the conc of HCl, it should be less than 7. Ignoring the inconsistency (check the indicator again, make sure there are no contaminants!), the pH tells you the concentration of in the water in molar. A low pH may mean a weak acid OR a low concentration.
To compare the actual strengths of acids, you need to compare two samples at the same concentration, i.e. 0.1M HCl and 0.1M CH3COOH.
In Units 1/2 (which is the extent of my present knowledge), you can calculate the pH of both acids/bases theoretically, however, this assumes that it ionises completely. CH3COOH, as we know, does not, so we cannot accurately calculate its pH theoretically with our Units 1/2 knowledge.
However, assuming that you actually do this in a lab, you would find that HCl has a lower pH than CH3COOH at the same conc, meaning that HCl is a stronger acid.