VCE Chemistry Question Thread

[qwerty101]

is NH3 a gas and NH4+ aqueous ?

[mahler004]

is NH3 a gas and NH4+ aqueous ?

NH3 can be a liquid as well. In solution there's an equilibrium between the acid and the base.

[qwerty101]

what is it mean to be "standardised"and is solution x is to be standardised in this titration?

thanks

[Kel9901]

is NH3 a gas and NH4+ aqueous ?

NH3 is a liquid at low temperatures (<0 degrees celsius), but is generally in the gas or aqueous phases (if in water/other solvent). NH4+ is always aqueous.

what is it mean to be "standardised"and is solution x is to be standardised in this titration?

thanks

standardised- means to have its concentration known. Standardising is done in order to allow the solution to be used to calculate concentrations of other solutions (think volumetric analysis).

For example, you already have a standard Na2CO3 solution, and use it to calculate the concentration of a HCl solution via volumetric analysis. Once this is known, you can then calculate the concentration of eg NaOH in a certain brand of oven cleaner. The process done to the HCl solution is known as standardising.

[qwerty101]

when creating a calibration curve for spectroscopy, should we force the line through the origin or do a line of best fit which may pass through (0,1) (as an example).

Also, if the points start at say 2,5 and end at 5,10 should we continue the line below 2,5 and above 5,10 or stop at these "end points" for the line of best fit?

[Kel9901]

when creating a calibration curve for spectroscopy, should we force the line through the origin or do a line of best fit which may pass through (0,1) (as an example).

Also, if the points start at say 2,5 and end at 5,10 should we continue the line below 2,5 and above 5,10 or stop at these "end points" for the line of best fit?

Don't force the line through the origin, do a 'line of best fit' per se.

You probably should continue as far as the axes go.

[RazzMeTazz]

In chromatography does the term 'mobile phase' refer to the sample as well, or does it purely refer only to the liquid/gas that is 'sweeping' the sample up the stationary phase?

Also for HPLC, my textbook states: "In HPLC the components are usually detected by passing the eluent stream through a beam of UV light. Many organic compounds absorb UV light." Does this mean, the term 'eluent' refers to both the solvent (used as the mobile phase) and the sample?

Thanks

[wunderkind52]

In chromatography does the term 'mobile phase' refer to the sample as well, or does it purely refer only to the liquid/gas that is 'sweeping' the sample up the stationary phase?

Also for HPLC, my textbook states: "In HPLC the components are usually detected by passing the eluent stream through a beam of UV light. Many organic compounds absorb UV light." Does this mean, the term 'eluent' refers to both the solvent (used as the mobile phase) and the sample?

Thanks

Mobile phase does not refer to the sample, purely the substance that moves the materials!
I think eluent refers to the solvent, but the term eluate refers to the both the sample and solvent. Not too sure about this one though. I'm sure someone more intelligent will correct me if I'm wrong

[qwerty101]

Can someone please explain exactly how colimetry works.  I understand that you pass the complement colour  (are we required to remember these?) , but exactly how does the solution absorb this light and why is that when the concentration (intensity of solution) increases, that absorbance also increases? is their absorbing species or something ?

thanks

[RazzMeTazz]

Would the retention time (Rt values) and retardation factor (Rf values) depend on the amount of that particular substance present?

So, if you were using sample and standard solutions and the Rt and Rf values of individual components in these solutions as a means of comparison, to determine which components are present your sample - would you have to ensure that the sizes of the sample and standards are similar?

Also, when measuring the distance components have traveled up the stationary phase, to calculate the Rf value, are you supposed to measure to the centre of the spot/band (formed by the component on the chromatogram) or the top or bottom of the spot/band?

Thanks

[qwerty101]

can someone please explain to me why/how two sharp points/bumps in a neutralisation curve occur?

[RazzMeTazz]

In gas chromatography, since the mobile phase used is an inert gas - does this mean that the mobile phase serves purely to sweep the substance across the stationary phase and that the Rt (retention time) values depends purely on the strength of attraction of the substance to the stationary phase, and does not depend on the substance adsorbing onto the mobile phase?

Thanks

[bae]

In gas chromatography, since the mobile phase used is an inert gas - does this mean that the mobile phase serves purely to sweep the substance across the stationary phase and that the Rt (retention time) values depends purely on the strength of attraction of the substance to the stationary phase, and does not depend on the substance adsorbing onto the mobile phase?

Thanks

Yes, exactly right. The mobile phase acts only as a carrier gas and does not have a role in separation.

[mahler004]

Would the retention time (Rt values) and retardation factor (Rf values) depend on the amount of that particular substance present?

So, if you were using sample and standard solutions and the Rt and Rf values of individual components in these solutions as a means of comparison, to determine which components are present your sample - would you have to ensure that the sizes of the sample and standards are similar?

Also, when measuring the distance components have traveled up the stationary phase, to calculate the Rf value, are you supposed to measure to the centre of the spot/band (formed by the component on the chromatogram) or the top or bottom of the spot/band?

Thanks

No, the amount of sample doesn't affect the retention time.

Conventionally, you measure from the middle of the peak or spot.

[chocolate.cake.1]

Hello

I have a question about redox reactions: According to my teacher's notes, "A gain of oxygen is oxidation and loss of oxygen in reduction". WHY? How does oxygen cause oxidation (loss of electrons)?