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: Sugar dissolving in hot water

  1. Mar 9, 2006 #1
    A spoonful of sugar is dissolving in hot water. The initial mass of the sugar was 7 grams
    and 5 minutes later the mass present had halved. The mass of sugar, S grams, at time
    t minutes after it was placed in the hot water can be modelled by S(t) = S0e^−kt, t ≥ 0.

    b Find the exact value of k.
    i sub it in the equation where i got :
    S(t)= 7e^-5k
    what does S(t) equals? does it equals to 35?

    thank you
  2. jcsd
  3. Mar 9, 2006 #2


    User Avatar
    Homework Helper

    So the model is

    [tex]s\left( t \right) = s\left( 0 \right)e^{ - kt}[/tex].

    You already know s(0), that's 7. That leaves one unknown parameter, k. But you have extra information, s(5) = 3.5, let's plug this in:

    [tex]3.5 = 7e^{ - k5} [/tex]

    Can you solve for k?
  4. Mar 9, 2006 #3
    oh okai i did not read the question properly. it do not see the halving part.
    thank you anywayz
  5. Mar 9, 2006 #4


    User Avatar
    Homework Helper

    You're welcome :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook