3.00 g of potassium hydroxide (KOH) pellets are put into a constant-pressure cal

Question

3.00 g of potassium hydroxide (KOH) pellets are put into a constant-pressure calorimeter containing 250 mL of water. The initial temperature of the water is 22.3°C. The mixture is then stirred until all of the KOH is dissolved. The final temperature of the solution is measured and recorded as 29.7°C. Calculate the heat of solution for potassium hydroxide (Hsoln). The specific heat capacity of the solution may be assumed to be the same as that of water since this is a dilute solution. Assume no heat loss from the calorimeter.

dH (solution) = -147 kJ/mol

What mass of KOH do you need to dissolve to reach a final temperature of  32 °C?

Explanation / Answer

m = 3 g of KOH

mol = mass/MW = 3/56.1056 = 0.05347 mol of KOH

V = 250 mL ; M = 250 g of water

Tw = 22.3°C

Tf = 29.7°C

Calculate Heat of solution

Q = m**C(Tf-Ti)

Q =250 * 4.184*(29.7-22.3) = 7740.4J

Qbase= -7740.4 J

mol base = 0.05347

Qsolution = -Qbase / n = -7740.4 /0.05347 = -144761.5485 J/mol = -144.761 kJ/mol

B)

What mass of KOH do you need to dissolve to reach a final temperature of  32 °C?

Q = m*Cp*(Tf-Ti)

HRxn = Q7n

HRxn*¨n = m*C*(Tf-IT)

-147000*mass/(56.10) = 250*4.184*(32-22.3)

solve for mass

mass = 250*4.184*(32-22.3)*56.10 / 147000 = 3.87 g required

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