Question

if you are given one element name and % by mass, how do you find the other element in the compound? give example plz

Answer 1

The percentage of elements in the compound is determined from the number of elements in the compound

Further explanation  

Proust states the Comparative Law that compounds are formed from elements with the same Mass Comparison so that compounds have a fixed composition of elements

$\tt \%A~in~A_xB_y=\dfrac{x\times Ar~A}{MW~A_xB_y}\times 100\%$

$\tt \%B~in~A_xB_y=\dfrac{y\times Ar~B}{MW~A_xB_y}\times 100\%$

Example

% Lithium (Li) in Li₂CO₃(lithium carbonate)

Ar of Li = 6.941 g/mol

MW of  Li₂CO₃  = 73.891 g/mol

$\tt \%Li=\dfrac{2.Ar~Li}{MW~Li_2CO_3}\times 100\%\\\\\%Li=\dfrac{2\times 6.941}{73.891}\times 100\%=18.79\%$