a) Formula mass of the acid
The equation for the reaction is
#"XCOOH + NaOH" → "XCOONa" +"H"_2"O"#
#"Moles of XCOOH" = 0.0215 color(red)(cancel(color(black)("dm"^3 color(white)(l)"NaOH"))) × (0.207 color(red)(cancel(color(black)("mol NaOH"))))/(1 color(red)(cancel(color(black)("dm"^3color(white)(l) "NaOH")))) × "1 mol XCOOH"/(1 color(red)(cancel(color(black)("mol NaOH")))) = "0.004 450 mol XCOOH"#
#"Molar mass" = "3.14 g"/"0.004 450 mol" = "706 g/mol"#
∴ Formula mass = 706 u.
b) Relative mass of #"X"#
#"Relative mass of X" = "relative mass of XCOOH" - "relative mass of COOH" = "706 - 45.02" = 661#
