[achibaby1974]

Ok. It's an online deal that allows you to submit your answers 5 times. I got all the other ones but I can't get the last one. It's just confusing. Answers have to be in scientific notation.

An empty Erlenmeyer flask weighs 254.2 g. When filled with water (d = 1.00 g/cm3), the flask and its contents weigh 470.4 g.

a.) What is the flask's* volume? cm3

(*ok. i'm getting confused because which flask are they talking about? the empty one, the one with contents...?For some reason, I just have a feeling that I have to do 470.4g - 254.2g = 216.2g. Why though? What does that give me?*)


b.) How much does the flask weigh when filled with chloroform (d = 1.48 g/cm3)? g




For part a.) my first guess

1.) Don't ask me what I was thinking. I did 216.2 x 1.48 = .004625 
umm, I don't know what happened. calculator error too. just me being all confused.
So the online response was: Did you accidentally divide or take the inverse in your calculation? cm3
Umm I have no idea what I was doing. So, stupid answer.   

2.) Then, I figured that since 216.2g is probably what mass they're wanting me to use (even though I don't know what it means) so that's the start for my next answer.

Since d=m/v, I know that d=1.00g/cm3 and I'm guessing that m=216.2 g.     
So I do 1.00g/cm3 = 216.2g/V so to find V= I divide M(216.2) by D(1.00g/cm3). That gives me 216.2g. Okay so in sic not, that's 2.162e2 (4 sig fig) right?
They say: Check the number of significant figures.

3.) Ok. By this time I'm disappointed. So, I do the same thing but with the contents full mass of 470.4g. I give them: 4.704e2. They say:  Your answer differs from the correct answer by 10% to 100%.

Ok. I've cleared my mind and I'm thinking now ok, maybe they ARE talking about an empty flask. 
So then the answer would be 254.2 cm3. So that would be 2.542e2, right?

Is this right or am I completely off? I'm really confused.





For part b.)

1.) I did 1.48/216.2 = .006846
They said: Your answer differs from the correct answer by orders of magnitude

2.) I thought oh, I forget to ut the answer in sci not. So I pot 6.846e-3
They said: Your answer differs from the correct answer by orders of magnitude

3.) well, cleared my head again and I tried 4.704e2 x 1.48 = 6.962e2
They said: Your answer differs from the correct answer by 10% to 100%.

Ok I have no idea. I'm just upset. I don't know whether b.) depends on the answer I get on a.)   


Thanks for any help at all.



   

[LQ43]

An empty Erlenmeyer flask weighs 254.2 g. When filled with water (d = 1.00 g/cm3), the flask and its contents weigh 470.4 g.

a.) What is the flask's* volume? cm3

(*ok. i'm getting confused because which flask are they talking about? the empty one, the one with contents...?For some reason, I just have a feeling that I have to do 470.4g - 254.2g = 216.2g. Why though? What does that give me?*)

2.) Then, I figured that since 216.2g is probably what mass they're wanting me to use (even though I don't know what it means) so that's the start for my next answer.

Since d=m/v, I know that d=1.00g/cm3 and I'm guessing that m=216.2 g.     
So I do 1.00g/cm3 = 216.2g/V so to find V= I divide M(216.2) by D(1.00g/cm3). That gives me 216.2g. Okay so in sic not, that's 2.162e2 (4 sig fig) right?
They say: Check the number of significant figures.

Is this right or am I completely off? I'm really confused.

full flask = 470.4 g
empty flask = 254.2 g

water = 216.2 g this is mass, you did this correctly but they want volume so use the density

but your units should be in cm³ and the density has only 3 sig figs
- set it the density units so that the g units cancel and you get cm³

216.2 g  x (1 cm³ / 1.00g) =  answer in what unit?

--> change to sci. notation with how many sig figs?

For part b.)

1.) I did 1.48/216.2 = .006846
They said: Your answer differs from the correct answer by orders of magnitude
  

think about the units of what they are asking for --> g and what you now have now: volume of the flask (from a) in cm³,

set it up so that the density units will cancel with the volume units.

vol (cm³)  x  density ( 1.48g/cm³) =  mass in grams (remember sci. not and sig figs)

using density is confusing for lots of people. just keep thinking what units need cancelling out and what units do you want left at the end. then you don't have to guess whether to multiply or divide the numbers, the setup does it for you

[achibaby1974]

Okay. My prof said that exact numbers, like many conversions, do not limit # of sig figs in calculation.

So, for part a)

the density of water is 1.00g/cm3 is exact right? So, how could it be included in sig fig calculations?

if there are only 3 sig figs then my answer would be 2.16e2! but now I'm a little iffy on what's exact numbers and what's not. 

ok. if 2.16e2 is the answer for a.) then i just do 2.16e2 x 1.48 = 319.68 = 3.1968e2
if rounded to i guess 3 sig figs then the answer would be 3.20e2.

it makes sense just not the sig fig part. but thanks so much for your help and thanks for not just telling me the answer. i really appreciate that because then i have to do the work and i learn better.

thanks!

[Alpha-Omega]

Unless I am not understanding the wording in the question...

In Part B) the question asks for the mass of the flask when filled wit the chloroform...you have the mass of the chlorofrm....do not forget to add the mass of the flask....the total weight will be the flask and the chloroform contents....

So do not round off untill you add in the flask....

[achibaby1974]

wait.

# of g of flask w/chloroform = 2.16e2cm3 x 1.48g/cm3 = 319.68 (g of chloroform)

                                      = 319.68 (g of chloroform) + 254.2 (empty flask)
                                         (b/c we are adding, we can only go over 1 decimal place right?)
                                      = 573.88, 5.7388e2, 5.7e2
                                      = so the answer is 5.7e2   



If 5.7e2 is the answer why did the online thing say my answers 6.846e-3 and 0.006846 differs from the correct answer by orders of magnitude? Wouldn't that mean that my numbers alright just not the exponent I'm using? 

[LQ43]

In Part B) the question asks for the mass of the flask when filled wit the chloroform...you have the mass of the chlorofrm....do not forget to add the mass of the flask....the total weight will be the flask and the chloroform contents....

So do not round off untill you add in the flask....

good point,

If 5.7e2 is the answer why did the online thing say my answers 6.846e-3 and 0.006846 differs from the correct answer by orders of magnitude? Wouldn't that mean that my numbers alright just not the exponent I'm using?  

5.7 E2 does not have the right number of sig figs. When adding, the answer should agree with number with the fewest # of decimal places . What is the answer? (don't forget units)
5.7 E2 is 5 orders of magnitude different from 6.4846 E-3 that is what a difference in exponents mean.

and it does feel good to work it out yourself (with a little help  )

[LQ43]

Okay. My prof said that exact numbers, like many conversions, do not limit # of sig figs in calculation.

So, for part a)

the density of water is 1.00g/cm3 is exact right? So, how could it be included in sig fig calculations?

1.00 g/cm3 is the density of water at a certain temperature (20oC) and  considered a measured value (most densities are) - and so could be considered to count in sig figs.

An exact value in conversion factor would be like 1000 g / 1 kg. Using this would not limit your calculation to 1 sig fig (because of the 1000g)

[achibaby1974]

5.7 E2 does not have the right number of sig figs. When adding, the answer should agree with number with the fewest # of decimal places . What is the answer? (don't forget units)
5.7 E2 is 5 orders of magnitude different from 6.4846 E-3 that is what a difference in exponents mean.

[/quote]

ok. I haven't tried the answers yet b/c I only have two more chances. But I still dont understand the sig fig.

2.16e2cm3 x 1.48g/cm3 = 319.68 (g of chloroform)

                                      = 319.68 (g of chloroform) + 254.2 (empty flask)
                                      = 573.88, 5.7388e2, 5.7e2
                                     
Fewest # of decimal places being 1 after the decimal right? so it should now have 4 sig fig? Im really confused about this stuff. It just doesn't make sense. The answer is 5.739e2? That's not one decimal place over. That's 3.  5.739-------   1st decimal place over.) 7,    2nd decimal place over.) 3    and    3rd decimal place over.) 9   


That's 3 decimal places not 1. I think I just don't understand this rule at all. Can someone explain the sig fig addition and subtraction rule? And what it means when dealing with sci not?

[achibaby1974]

ok. after much thought i think i got it now. I add the 319.68 + 254.2 = 573.88
however, since it 254.2 has the least decimal places I have 4 sig figs total right?

So my answer would be 5.739e2? Right? This now sounds semi-right.

[achibaby1974]

okay i submitted my answers for a.) and b.).

a.) 2.16e2
Correct.

b.) 5.739e2
Incorrect. Check number of significant figures.

So, now I'm guessing that since I forgot

2.16e2 cm3 (1.48g/cm3) = 319.68
here technically there would only be 3 sig figs.

but to avoid round off error I keep it in mind and continue to add 319.68 + 254.2 = 573.88
That's 5.7388e2. But since I added and I have to keep in mind that 319.68 only had 3 sig figs and 254.2 had 4, my answer would only have 3 sig figs. So the answer would be 5.74e2!

Right?

[LQ43]

yes I would agree, even if you rounded 319.6 to 320.g  the answer would still be 5.74 E2 g