StudentQns' chemistry questions

StudentQns SG

Hi my other thread expired so I’m writing a new one.

boiling point of salt water and misible liquid mixtures
They say that when I add an impurity n it dissolves in the solvent, its melting point will decrease while boiling increases. But when performing simple distillation the thermometer reads 100 degree celcius. So what is the link between this n the raised boiling point?

Is the mixture at eg 120degree while the water vapour that rises up is 100 degrees? I don’t really understand this concept well.

Using the same understanding, when I put water and ethanol, how will the boiling point change? Since it is like a liquid with impurities will the increase in boiling point n decrease n melting point be in relation to the substance with the lower boiling or melting point?

frekiwang SG

You are a critical thinker as I used to be, I like it ^_^

To answer your question:

It is true that the boiling point of salt water is slightly higher than 100 degree C (for example, 101.4 degree C for sea water). There are two things that you may want to consider in this situation.

1. 100 degree C on theometer is an estimation only, we are not keen in finding the difference between the actual reading and the value 100. In fact, even you are boiling pure water in a sealed flask, the boiling point will not be exact 100 degree C as the pressure is usually not equal to the atomspheric pressure.

2. Do remember, theomometer is placed above the salt water but not in the salt water in either type of distillation(IMPORTANT). Therefore, it is used to measure the temperature of the water vapour but not the boiling salt water. Since the water vapour is 'pure' as it does not contain salt, the temperature will drop to 100 degree C immediately after leaving the surface of the boiling water (as water vapour will not be able to gain heat from the surrounding since the temperature of the surrounding will not be higher than 101.4 degree C), so the theomometer will read 100 instead of 101.4 assuming the atomspheric pressure applies.

atomos SG

Another way we can look at it is that the vapour is pure water in gaseous form.

The temperature measured is actually when the vapour condenses onto the thermometer bulb.

For pure water (at 1 atm), condensation point = boiling point = 100 degree C.

SBS2601D SG

No wait.

It is true that pure water boils at 100 deg, but water vapour is free to hit temperatures above 100 deg.

The excess energy is used to expand and heat the gas rather than break the hydrogen bonds as in boiling and evaporation.

atomos SG

Yup, agreed.

Just that the question is in the context of the thermometer's role in a simple distillation setup.

StudentQns SG

Thanks for the help all of u guys! :-) being a critical thinker ain’t very good though. Stresses me out whenever I meet with a concept which is difficult.(mostly caused by me thinking too much n going too deep) thanks for all the help forumers!

StudentQns SG

Hi, I now have a simple question regarding QA. When testing for unions like chloride, iodides sulfates, why do we have to add HNO3 first?

Thanks for the help!

frekiwang SG

Take chloride as an example,

If we only add barium nitrate(aq) and observe white ppt formed, we cannot conclude the white ppt is definitely barium chloride(s) since barium carbonate is also a white ppt. By adding nitric acid, white ppt is confirmed to be barium chloride as barium carbonate will react with nitric acid to form colourless solution and carbon dioxide gas.

Darkness_hacker99 SG

Hi, I now have a simple question regarding QA. When testing for unions like chloride, iodides sulfates, why do we have to add HNO3 first?

Thanks for the help!

Not in your syllabus. Practical will not ask this.

But it's good to be curious.

You must first acidify the solution why dilute acids to remove any interfering ions which may affect the test results. Like what frekiwang mention, it's to make sure the white ppt. is barium chloride.

StudentQns SG

Oh ok thanks for the help :-)

StudentQns SG

Not in your syllabus. Practical will not ask this.

But it's good to be curious.

Sorry seems like editing posts will result in a bug to occur. I was also told that it was to remove any cations present in it too. So how does the HNO3 participate in this? Thanks for the quick response u guys rock! :)

atomos SG

Like what frekiwang mentioned,

the HNO3 (acid) will remove the carbonate ions (acid + carbonate --> carbon dioxide), which would otherwise react with barium to form barium carbonate (white ppt)

StudentQns SG

Oh ok thanks for the help! Smile
Also then why when I do titration using a pH meter, the pH will drop all the way to 4 from 10 exactly as a certain volume?

http://www.chemguide.co.uk/physical/acidbaseeqi…

From here they say that some ions are acidic. What does it mean by that? I thought that when something is acidic is because of the H+ ion? Like no of moles of H+ ions/volume of water.

Thanks for the help guys!

Darkness_hacker99 SG

Oh ok thanks for the help! Smile
Also then why when I do titration using a pH meter, the pH will drop all the way to 4 from 10 exactly as a certain volume?

http://www.chemguide.co.uk/physical/acidbaseeqi…

From here they say that some ions are acidic. What does it mean by that? I thought that when something is acidic is because of the H+ ion? Like no of moles of H+ ions/volume of water.

Thanks for the help guys!

Do titration like a pro using a pH meter.

http://www.youtube.com/watch?v=YDzzMcrdyB4&feature=pyv

Yes, you will notice a sudden jump in pH. If you were to draw it in a titration curve, it'll look like 2:37 in the video.

Yes, acidic/basic due to H+/OH- ions. Chemical do exhibits some physical properties. Acidic/basic could be due to net charge which will be learn in A level.

UltimaOnline SG

Yes, acidic/basic due to H+/OH- ions. Chemical do exhibits some physical properties. Acidic/basic could be due to net charge which will be learn in A level.

Yes, 'A' level students should be able to figure out which are the two different types of cations are acidic (relative to water), and be able to explain why. Hint : one type organic, one type inorganic.

StudentQns SG

oh ok. So at O level if they ask me what is the pH of the solution after neutralisation occurs. I just say 7?

EiN930 SG

oh ok. So at O level if they ask me what is the pH of the solution after neutralisation occurs. I just say 7?

Yes. Also, usually the qn will state the addition of strong acid (like sulfuric acid) to a strong base (like NaOH) or vice versa.

Just for your information, if you add a weak acid to a strong base, or a strong acid to weak base, the pH will not be 7.

SBS n SMRT SG

Strong acid - weak base titration, end point pH will be less than 7

Weak acid - strong base titration, end point pH will be more than 7

This is also subject to the salt formed (which is out of O level syllabus)

In addition, pH may also vary with dilution and hence, the pH is assumed when 1 dm ^ 3 of solution is used and the acids / bases are 1 M (moldm^-3)

SBS n SMRT SG

Answer (Hopefully correct):

CH3COO-Na+ --> CH3COO- + Na+

CH3COO- + H20 <--> CH3COOH + OH -

Hence, [OH-] > [ H +], pH more than 7

Al3+ will undergo acid hydrolysis to form a solution of pH of approx 3, applies to Cr / Fe also

(Equation too complicated, dun want to type out)

UltimaOnline SG

Answer (Hopefully correct):

CH3COO-Na+ --> CH3COO- + Na+

CH3COO- + H20 <--> CH3COOH + OH -

Hence, [OH-] > [ H +], pH more than 7

Al3+ will undergo acid hydrolysis to form a solution of pH of approx 3, applies to Cr / Fe also

(Equation too complicated, dun want to type out)

Correct for the inorganic metal ion (though a more complete answer would include both equations and explanation).

Not so correct for the organic ion, because my question was "which organic cations are acidic" and not "which organic anions are basic".