Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook