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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: MITaylor on September 29, 2014, 08:54:40 PM

Title: Energy to Heat Up Water
Post by: MITaylor on September 29, 2014, 08:54:40 PM

How much energy is required to heat up about 500g of water from 20°C to 85°C?"

Using the formula Q=SmΔT, I'm guessing I would multiply 4.184 (the specific heat capacity of water) J/g°C * 500g * 65°C, which would give me Q≈100,000 J, assuming that the limiting number of significant figures would be the "5" in 500g.  Did I do that correctly?

Title: Re: Energy to Heat Up Water
Post by: Borek on September 30, 2014, 03:13:04 AM

While technically you are right, I don't think that's a reasonable answer. I would put it as "around 140 kJ".

Significant figures are a lousy way of dealing with the accuracy of calculations, especially when there is a single digit involved.

In general 500 g means something between 450 and 550 (otherwise it would be listed as 400 g or 600 g, or something else).

That in turn means amount of heat required is something between 122 kJ and 150 kJ. 100 kJ - despite having a correct number of significant figures - is definitely outside of this range.