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Chemistry review questions-calculations

  1. Jun 17, 2006 #1

    Aya

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    Hi, I have some reviw questions for the upcomming chemistry exam, the problem is I don't have the answers so can some one read throught my work and check if it is wright? Thanks.
    ***
    1. An anaysis of a valitile liquid shoed that it wsa composed of 14.4%C, 2.37% H and 83.49% Cl. If 4.25 g of the liquid was vaporized and occupied 628 mL at 65 degrees celcuios and 112 kpa, what would be the molecular formula of the compund?
    carbon
    14.4% x 100g
    = 14.4 g

    m=14.4 g
    n=1.199 mol
    Mm=12.01 g/mol

    hydrogen
    2.37% x 100g
    =2.37g

    m=2.37 g
    n=2.347mol
    Mm=1.01 g/mol

    Cholrine
    83.49% x 100g
    =83.49g

    m=83.49 g
    n=2.355mol
    Mm=35.45 g/mol

    so...

    1.199 / 1.199 =1
    2.37 / 1.1.99=2
    2.355/ 1.199=2

    Impirical Formula
    C1 H2 Cl2

    # mol of Impirical formula
    PV=nrt
    n=PV/rt
    n=(112Kpa)(0.628L)/ 8.314kpaL/molL (388k)
    n=0.025 mol

    Molar mass of Impreical formula
    Mm=m/n
    Mm=4.25g/0.025 mol
    Mm=170 g/mol

    Molar mass of ( what is this the molar mass of???)
    C1 H2 Cl2
    84.93

    Find a Factor

    170g/mol/ 84.93 g/mol
    =2

    Molecular Formula
    2(C1 H2 Cl2)

    C2 H4 Cl4




    ***
    2. A gas has a volume of 40.0 ml at 40 degrees celcuios and 95 Kpa. What will be its volume at STP?

    gas
    40.0 mL = 0.04L
    40 degrees celcioue=313 K
    95 kPa=P

    n=rt/pv
    n= 8.134(313)/95(0.04)
    n=27.3890 mol

    v=nrt
    v=27.3809(8.134)(273)/101.3
    v=613.4948
    ***
    3. How many grams of sodium and required to product 2.24 L of Hydrogen gas, measured at 25 degrees celcuoiu adn 110 Kpa, according to the following reaction?

    2Na + 2 H2O ---> 2 NaOH + H2O

    n=rt/pv
    n= 8.314(298)/110(2.24)
    n=10.055

    m=nMm
    m=20.11 (22.99)
    m=462.3289

    B)calculate the volume of H2(g) produced when 54 g of Na(s) reacts with an excess of water at STP.

    moles of Na
    n=m/Mm
    n=54/22.99
    n=2.348

    moles H
    3.348/2
    1.174

    Volume
    v=nrt/p
    1.174(8.314)(273)/101.3
    v=2.791

    ***
    4. An aquois solution has a volume of 2.0L and contains 36.0 g of glucose. If the molar mass of glucose is 180 g , what is the molarity of the solution?

    molar mass of glucose
    n=m/Mm
    n=36.0/180
    n=0.2

    ...what next?
    ***
    5. How would you prepare 100ml of 0.40 mol/L MgSO4 form a stock solution of 2.0 mol/L MgSO4?
    n=0.100L/0.40 mol/L
    0.25

    v=n/c
    v=0.25/2.0
    v=0.125L
    add 0.125L of MgSO4
    ***
    6. A solution contains 5.85 g of sodium chlride dissolved in 5.00 x 10^3 ml of water. What is the concentration of the sodium cholirde solution?
    n=M/Mm
    n=0.0568 mol


    C=n/v
    0.0568/5
    =0.01136 mol/L
    ***
    7.what mass of potassium hydroxide is required to prepare 6.00 x 10^2 ml of a solution with a concentration of 0.225 mol/L
    m=cv
    0.225(0.5)
    0.135mol

    ***
    8. What volume of 0.500mol/L sodium hyroxide solution can be prepared form 10.0 ml of a 6.00 mol/L solution?
    C1V1=C2V2
    6(0.01)=0.500(v2)
    0.12=v2

    ***
    9. What is the mass of hydrochoric acid that is present in 500 mL of a solution containg 3.50 mol/L of HCl(aq)?
    n=cv
    3.50(0.5)
    =1.75 mol

    m=nMm
    1.75(36.46)
    63.805g

    ***
    10. Passing a park throught a mixture of hydrogen gas and oxygen gas produced water

    a) calculate the mas of hydrogen needed to completly convert 4.00 g of oxygen into water

    H2 + O = H2O

    moles of O
    n=m/Mm
    4.00/18.025
    0.22

    moles of H
    0.22 mol

    mass of H
    m=nMm
    =0.22(1.01)
    =0.224

    b) calcute the number of moles of oxygen required to react with 12.5 moles of hydrogen gas
    I dont know how to do this one
    c)calculate the number of moles of water produced when 4.00 g of oxygen are used

    I dont know how to do this one

    ***

    Thanks for reading pleas post any corrections and solutions to the problems I don't know how to do. this is practise for an exam so pleas help!!!! :smile:
     
  2. jcsd
  3. Jun 18, 2006 #2

    Hootenanny

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    Staff Emeritus
    Science Advisor
    Gold Member

    HINT:

    [tex]\text{M} = \frac{\text{moles}}{\text{volume}}[/tex]

    You have almost already written the answer in your post, look again at the equation you written, i would just like to correct it however;

    [tex]H_{2}_{(g)} + \frac{1}{2}O_{2}_{(g)} \rightarrow H_{2}O_{(l)}[/tex]

    Now, all the answers you need are in the above equation. :smile:
     
    Last edited: Jun 18, 2006
  4. Jun 18, 2006 #3

    Aya

    User Avatar

    4.
    M=Mol/vol
    M= 0.2mol/2.0L
    M=0.1mol/L
    ***
    The teacher said we should only balance with whole nubbers so would it be

    2H2 + O2 = 2H2O....?

    moles of H
    0.22 mol
    sooo... moles of O is also 0.11mol?

    c)calculate the number of moles of water produced when 4.00 g of oxygen are used

    2H2 + O2 = 2H2O
    2 : 1 = 2

    oxygen
    n=M/mm
    n=4.00g/16.0
    n=0.25mol

    Water
    0.25mol x 2
    =0.5 mol

    Are these ones right, and is everything elce right?

    Thanks for your help!
     
  5. Jun 18, 2006 #4

    Hootenanny

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    Staff Emeritus
    Science Advisor
    Gold Member

    Spot on.
    If your teacher restricts you to using whole numbers, then yes that is correct, however using half a diatomic molecules is acceptable.
    In you original post you said, 12.5 mols of hydrogen.
    Yes, that's spot on! I'm afraid I haven't checked the other as I haven't really got time, my apologies; but if you check back later I'm sure someone will have obliged.
     
  6. Jun 18, 2006 #5

    Aya

    User Avatar

    ^ Oh, ok thanks for all your help!
     
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