# The Naked Scientists Forum

### Author Topic: chem h.w. help  (Read 2748 times)

#### gill

• First timers
• Posts: 1
##### chem h.w. help
« on: 04/10/2005 00:40:50 »
hi this is my first time in this forum..i am doing a lab for school that involves dilution.  im supposed to also measure the absorbance of a solution.  im using a chemtrix colorimeter to measure the solution's absorbance and i dont know how many decimal places i should record the value to.  for one of the absorbance value i put .10 . is that about right, putting 2 numbers after the decimal place? getting the right amount of sig figs is very important in this lab..

#### chris

• Neilep Level Member
• Posts: 5295
• Thanked: 61 times
• The Naked Scientist
##### Re: chem h.w. help
« Reply #1 on: 04/10/2005 16:06:52 »
The number of significant figures will be dependent upon the accuracy and precision of each of the other steps in the experiment.

If you weighed out a sample of a salt correct to 2 significant figures - e.g. 1.1 g - but then reported the absorbancy to 5 significant figures, the additional 3 sig figs are meaningless because the greatest accuracy you can achieve is to 2 sig figs.

Chris

"I never forget a face, but in your case I'll make an exception"
- Groucho Marx

#### Ylide

• Moderator
• Hero Member
• Posts: 905
##### Re: chem h.w. help
« Reply #2 on: 08/10/2005 19:28:13 »
Chris is completely correct in that your overall precision is limited by your least number of significant figures anywhere in the calculation.

As for the reading on your colorimeter, the number you record will be dependent on if it's an analog or digital gauge that you read the absorbance from.  If it's analog, you estimate the last digit and give yourself a +/-5 error in that digit's place.  For instance, if you the gauge reads halfway between 1.2 and 1.3 (purely hypothetical examples) you would record 1.25 with a +/- 0.05 error.  This gives you three sig figs to work with.

If you have a digital readout, just record the number as shown and assume +/- 2 in the last significant digit as your error.

This message brought to you by The Council of People Who Are Sick of Seeing More People

#### The Naked Scientists Forum

##### Re: chem h.w. help
« Reply #2 on: 08/10/2005 19:28:13 »