chem help

[jaques]

what does calibrate the graph mean?

[TrueTears]

I assume you mean calibrate the graph as in draw a calibration line? In most cases just join the dots, this would just be a straight line.

Your graph should have peak height on the y axis and concentration (in ppm mostly) on the x axis.

[Gloamglozer]

I think TrueTears is right.

Although sometimes, if you don't get a perfect straight line through all the points, you will have to draw a line of best fit.

[khalil]

Q8.
During the preparation of the standard solution shown in Figure 3.4, why is water added to the level of the calibration mark on the flask after the solid has dissolved, rather than before?
A8.
When a substance is dissolved, there is often a slight change in volume due to the attractions between the solute and solvent particles. Since molar concentration is measured in mole of solute per litre of solution, it is necessary to accurately measure the volume of solution rather than the volume of water used.

can someone explain this in simpler terms?

[TrueTears]

Q8.
During the preparation of the standard solution shown in Figure 3.4, why is water added to the level of the calibration mark on the flask after the solid has dissolved, rather than before?
A8.
When a substance is dissolved, there is often a slight change in volume due to the attractions between the solute and solvent particles. Since molar concentration is measured in mole of solute per litre of solution, it is necessary to accurately measure the volume of solution rather than the volume of water used.

can someone explain this in simpler terms?

Ok, so basically everytime something dissolves, it is forming a bond with the water molecules. Whatever that solid may be, when it dissolves in water the volume will slightly change due to the bonds it forms with the water molecules.

Now since concentration is defined as , ie as the answer states mole of solute per litre of solution. As 'v' changes the concentration also changes, so you must wait until all the solid has dissolved then add water to the calibration mark in order to achieve a more accurate solution.

[khalil]

Ok thanks but why do they say we must measure the volume of solution rather than the volume of water used?

[TrueTears]

Ok thanks but why do they say we must measure the volume of solution rather than the volume of water used?

because the solid is dissolved in the water, that's why its called a solution

If there were no solids in it, then its just plain old water

If you didn't dissolve the solid, and just poured the water up to the mark, then you would be actually only measuring the amount of water in the flask. (Because the solid hasn't formed bonds yet with the water molecules so the amount of water in the flask does not change, here we can just ignore the space that the solid takes up, we assume it is negligible) Therefore by dissolving it, we are actually making the water and solid into a "whole" solution.

[methodsboy]

what does calibrate the graph mean?

To calibrate is to:
-determine the conc and absorbance of the standards
-plot the points on a absorbance vs. conc graph
-choose the line of best fit
- you can then use this line to determine the conc of substances with a known absorbance