Sunday
May 1, 2016

# Homework Help: Value of lava mass

Posted by Freddo on Saturday, May 5, 2007 at 11:06am.

This question has been posted before, and I amlooking for critque of my answer ..Thanks !!!!

The specific heat capacity of the lava is 1600 J kg^-1 degres celsuis ^-1. when all the erupted lava cools from the eruption temperature of 1050 degrees Celsius to 15 degrees celsius, the heat released is 2 x 10 ^15 J.
calculate a value for the total mass of lava that erupted. give your answer in scientific notation.

Is this the answer or can you help me where I go wrong !!

1600 J kg -1 °C -1 x 2 x 10 15 J x 1035 °C = 3.312 x 10 21 kgs

?????????????????

You haven't substituted correctly.
The equation is
q = mass x specific heat x delta T
q = heat released = 2E15 J.
mass = solve for this.
specific heat = 1600 J/kg*C
delta T = 1035
To solve for mass,
mass = q/(specific heat*1035) = ??kg.

Thank you !!!!

is this right

1.29375 x 10 15 kgs in sci notation ????

I get a slightly different number as well as a different exponent.
mass = q/sp.h.*delta T
mass = 2E15/1600*1035
mass = 1,207,729,468.60 kg.
In scientific notatin I would write that as
1.207 X 10^9 kg. Does your teacher pay attention to significant figures? Technically, there is only one allowed in the answer (the 2E15 has only one s.f.; i.e., the 2 unless the original problem was 2.00E15 or something like that) which would make the answer, with due regard to number of s.f. as 1E9 kg. Check my arithmetic. Check my thinking.

I have tried to get the same total as you...but still come back with the answer given above...

Divide by specific heat (1600). You should get 2E-15/1600 = 1.25E+12.
Now take that number and divide by the change in temperature (1050-15=1035).
1.25E+12/1035 = 1.207E9.

I was multipling the last figure instead of dividing..thank you very much !!!

I appreciate your time and trouble in helping me make sense of these questions.

Hi Freddo,the answer is 1*E9kg.One sig fig!

Be sure and watch the number of significant figures if your prof does. My earlier answer of 1E9 kg is the correct one; I wrote more s.f. in my last post because we were checking your calculator proficiency. Glad to be of help.