Chem Help!!

[orangez]

Hey fellow Chem buddies, I need help with a few questions.

Here's the data:
m(ash) = 0.055 g

1. In this experiment, the iron in the ash from the food was dissolved in 15mL of liquid (10mL of HCl acid and 5mL of water). A 5mL volume of this solution was then mixed with 5mL of potassium thiocyanate and the absorbance measured.
 a) What is the concentration of iron in the 15mL of liquid, in mg/L? (the mass should be 0.055g)
 b) Assume the 15mL of liquid weighs 15g (ie the density is 1.0g/mL). Using your answers to a, calculate the mass of iron in this volume of liquid. This is the mass of iron present in the 2.5g food sample.

I've already figured out part a, but part b remains a mystery. Are you supposed to use the density equation, and so the density is the concentration?

Thanks!!

PS these questions are part of an experiment -- and not all data has been included!!

[Mao]

sorry it doesnt quite make sense...
the assumption that density of 1g/mL should have been made when calculating in part a when you have fround the mg/L. (part b also relies on this assumption, but this is not where the assumption be initially made. calculation of concentrations in w/v depends on this assumption, which occurs earlier on)

what type of instrument was used? (i'm assuming AAS), and how did you get the 0.055g? (did u get that from the calibration curve?)

[orangez]

Yeah, it's a stupid question. It was a colorimetry experiment. Hmm... I'm thinking that I should tell you what happened. I guess I will then:

The experiment was about finding the % of iron in spinach. Firstly, we chopped it up then placed it in a crucible, to heat it up until it turned into ashes. Afterwards, the ashes were dissolved in HCl solution, and after some stirring, I think (I don't have the procedure with me) 5mL of distilled water was added. 5mL of this solution was then taken and mixed with 5mL of potassium thiocyanate. This produced a reddish colour, where we would estimate the % of iron by using standards (my group got ~0.0020%.

[orangez]

Oh yeah, the 0.055g was obtained from measuring the mass of ash --> m(ash + crucible) - m(crucible) = m(ash)

[Mao]

the iron ion forms a ligand complex with water and thiocyanate attracted to it, which is the colour you see.

"~0.0020%." what unit is this in? (%w/v?)
assuming it is in %w/v (g/100mL)

the the concentration of iron in the 5mL before thiocyanate was added is then ~0.0040%, which is the same as the concentration of the 15mL solution.
0.0040% w/v = 0.0040 g/100mL = 4.0 mg/100mL = 0.40 mg/L

the "%w/v" is based on the assumption that the solution has a density ~1.0 g/mL

[Fe³⁺]= 0.40 mg/L
m(Fe³⁺)=0.40 mg/L * 15 mL = 0.0060 mg  (total iron in the 15mL, assuming that is all your ashes that are dissolved, and that is all the ashes from the 2.5g spinach sample)

[orangez]

Thanks Mao!! Your help is very much appreciated ^^.