Possible webpage title: Calculating Heat Needed for a Reaction (Q10)

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The discussion focuses on calculating the heat required for a reaction using the formula Q = m_w*c_w*deltaT + m_w*L_f + m_w*c_i*deltaT. The user presents their calculation, arriving at a value of 209,100 J, but notes that this does not match any provided answer choices, which are in kcal. Participants emphasize the importance of unit conversions and suggest verifying the calculation steps to ensure accuracy. The correct approach involves careful attention to units and values, particularly when converting between joules and kilocalories. Overall, the conversation highlights common pitfalls in thermodynamic calculations and the need for precision in unit handling.
jack1234
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Heat that needed(q10)

For this question:
http://tinyurl.com/2leprw

Followed is my working

Q=m_w*c_w*deltaT + m_w*L_f + m_w*c_i*deltaT
=m_w[
c_w*deltaT +
L_f +
c_i*deltaT
]
=2*(10^3)g [
1 cal/(g*celcius) * 20 celcius +
(3.33*(10^5)*0.2389/1000)calories/g +
0.5 cal/(g*celcius) * 10 celcius
]
=209100J

And this is not one of the choices...
Can anybody kindly pointed out what is my mistake?
 
Last edited:
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Well the choice of answers are in kcal rather than J.

Q=m_w*c_w*deltaT + m_w*L_f + m_w*c_i*deltaT

is correct. M = 2000 g (2 kg) so

Q = 2000 g ( c_w*c_w*deltaT + L_f + c_i*deltaT)

= 2000 g ( 1 cal/(g-°C) * 20°C + 79.7 cal/g + 0.5 (cal/g°C) * 10°C)

Be careful with conversions, and please show units.
 
Using the values you've given even I don't get an answer that's on the sheet.
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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