Mathmatical formula in cells

Hi,

i'm a biology studenttring to add a formula to a cell in my spreadsheet. I'm trying to find the log10 of a value and then times it by the same value... eg. log10C4*C4.

The formula i've got at the moment is =sum((log10(C4))*C4). Although this gives me a value its not the same as doing it on a calculator.

I should at this point mention that maths is not my strong point so i may well have missed out something obvious. However, if its the excel formula that i'm using could someone please tell me what i'm doing wrong.

Thanks in advance,
Matt