1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Basic Chemistry Calculations

  1. Sep 8, 2008 #1
    Hello. I just started a first year chemistry course after switching to the sciences from the arts, and wanted to double-check some equations with you because this is a little new to me. I'm good with the qualitative stuff, but the quantitative parts come a little harder.

    Q1: A sheet of metal is 93.3 mm wide and 40.6 mm long. If it weighs 4.877 g and the density of the metal is 1.74 g/cm3, what is the thickness of the sheet (in mm)?

    My answer: 4.877g / 1.74g/cm^3 = 2.80cm^3

    93.3mm x 40.6mm = 3780mm (3 sig figures)

    2.80cm^3 / 3780mm = 0.740mm

    93.3mm x 40.6mm x 0.740mm = 2.80cm^3

    Thus, the sheet is 0.740mm thick.

    Q2: If PV = [gR(T+273.15)]/M, solve for M when P = 334, V = 0.350, g = 0.274, R = 62.37, and T = 39.

    My answer:

    PV = [gR(T+273.15)] / M

    (334)(0.350) = (0.274) (62.37) (312.15) / M

    116 = 5330 (3 sig figures) / M

    116 / 5330 = M

    0.0217 = M

    Q3: An ore contains 42.3 % of the mineral ilmenite, FeTiO3, which is a source of the element Ti. How much ore must be processed in order to obtain 41.0 kg of Ti?

    Molar masses

    Fe = 55.85g
    Ti = 2004.4g
    O = 16.00g x 3 = 48.00g

    total = 308.2g

    308.2g / 204.4g = 1.50

    41.0kg x 1.50 = 61.5kg

    100 / 42.3 = 2.36

    61.5 x 2.36 = 145 kg of ore are needed to obtain 41.0kg of thallium.

    Thank you all in advance.

    M.
     
  2. jcsd
  3. Sep 9, 2008 #2

    Borek

    User Avatar

    Staff: Mentor

    First looks OK. Second looks wrong (don't round down intermediate values, check your math). Third looks wrong (check molar mass of Ti).
     
  4. Sep 9, 2008 #3
    Opps. Got the element wrong for number three. Should be...

    Q3: An ore contains 42.3 % of the mineral ilmenite, FeTiO3, which is a source of the element Ti. How much ore must be processed in order to obtain 41.0 kg of Ti?

    Molar masses

    Fe = 55.85g
    Ti = 48.77
    O = 16.00g x 3 = 48.00g

    total = 151.7g

    308.2g / 151.7g = 3.168

    41.0kg x 3.168 = 129.9kg

    100 / 42.3 = 2.36

    129.5 x 2.36 = 300 kg of ore are needed to obtain 41.0kg of titanium.

    For number 2, is my mistake that I rounded before I got to a final answer?
     
  5. Sep 9, 2008 #4

    Borek

    User Avatar

    Staff: Mentor

    One of mistakes. Check your math.
     
  6. Sep 9, 2008 #5
     
  7. Sep 10, 2008 #6
    Ok, let me work through this and see if I can figure out where I went wrong.

    Q2: If PV = [gR(T+273.15)]/M, solve for M when P = 334, V = 0.350, g = 0.274, R = 62.37, and T = 39.

    My answer:

    PV = [gR(T+273.15)] / M

    (334)(0.350) = (0.274) (62.37) (312.15) / M

    The left side actually works out to 116.9, and I won't round the right side.

    116.9 = 5334.5 / M

    Moving 5334.5 to the left side, it goes from the numerator to the demoninator.

    116.9 / 5334.5 = M

    0.0219 = M (3 signifigant figures)

    I hope I got that right, because I have another one =P

    Q4: A piece of nickel wire has a diameter of 0.505 mm. If nickel has a density of 8.90 g/cc, how long (in meters) should you cut a piece of wire to obtain 0.0247 moles of nickel?

    My answer: since a wire is a cyllinder, then the volume = πr2h

    r = 0.505 / 2 = 0.252

    Nickel weighs 58.69 g/mole, so there are 58.69g/mole x 0.0247 moles = 1.45g of the wire needed to obtain the correct length.

    So v = π (0.252)2h

    Since densty is 8.90 g/cc, then its volume is 1.45g / 8.90g/cc = 0.163 cc

    So 0.163 cc = π (0.252)2h

    h = 0.163 cc / π (0.252) 2

    h = 0.817cm of wire is needed

    I’d like to thank you all for giving me a hand with this. I know this is something you have to practice, but I’m afraid of practicing it wrong and forming bad habits.
     
  8. Sep 10, 2008 #7

    Borek

    User Avatar

    Staff: Mentor

    Why have you moved M from denominator to numerator? What you are doing you are dividing both sides by 5334.5, it cancels out on the right - but M stays where it was!

    Ni wire looks OK to me.
     
  9. Sep 10, 2008 #8
    But, if 116.9 = 5334.5 / M, then doesn't 116.9 / 5334.5 = M? Both sides are divided by 5334.5, so the left side becomes 116.9 / 5334.5 and the right side becomes (5334.5 / M) / 5334.5, which cancels out to just M.
     
  10. Sep 11, 2008 #9

    Borek

    User Avatar

    Staff: Mentor

    No, it cancels out to 1/M.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Basic Chemistry Calculations
  1. Basic Chemistry (Replies: 3)

Loading...