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Homework Help: How do you convert Density into moles?

  1. Oct 9, 2008 #1
    1. The problem statement, all variables and given/known data
    When 10.00 mL of a solution of strong acid is mixed with 100.00 mL of a solution of weak base in a coffee-cup calorimeter, the temperature falls from 25.7oC to 20.7oC. Determine q for the acid-base reaction, assuming that the liquida have densities of 1.00 g/mL and the same heat capacities as pure water.

    2. Relevant equations
    So I see that you're supposed to use the Q = (Change in T)(N)Molar Heat Capacity of H2O) but they've given the density of water so we're supposed to convert that into moles. I'm obviously doing something wrong, but does anyone know how to convert the density into moles?

    Density = Mass (g) / Volume (L)

    moles = Mass(g) / Molar Mass (g/m)
  2. jcsd
  3. Oct 10, 2008 #2


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    Staff: Mentor

    You can't convert just density into moles. Density is an intensive property, number of moles is an extensive property, they are not related.

    However, you are given extensive property - that is volume. You will need that.
  4. Oct 13, 2008 #3
    I think the easiest way to solve this problem is to avoid moles alltogether. Specific heats are _usually_ expressed as J/Kg/C, so moles aren't required unless you want them to be.

    You know what your total mass of solvent is and it's specific heat, so finding the energy released with the change in temperature should be trivial.
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