Your tablet contains 0.33 g of aspirin, and has a percent composition of aspirin of 66%.
The key to this problem is the balanced chemical equation for this neutralization reaction
C_9H_8O_(4(aq)) + NaOH_((aq)) -> C_9H_7O_4Na_((aq)) + H_2O_((l))C9H8O4(aq)+NaOH(aq)→C9H7O4Na(aq)+H2O(l)
Notice the 1:11:1 mole ratio that exists between aspirin and sodium hydroxide. This tells you that, in order for a complete neutralization to take place, you need equal numbers of moles of each compound.
Since you know the volume and molarity of the sodium hydroxide solution used in the titration, you can determine the number of moles of base used
C = n/V => n = C * VC=nV⇒n=C⋅V
n_(NaOH) = "0.10 M" * 18.30 * 10^(-3)"L" = "0.00183 moles"nNaOH=0.10 M⋅18.30⋅10−3L=0.00183 moles NaOHNaOH
This means that the tablet contained
0.00183cancel("moles"NaOH) * "1 mole aspirin"/(1cancel("mole"NaOH)) = "0.00183 moles" C_9H_8O_4
Use aspirin's molar mass to determine how many grams would contain that many moles
0.00183cancel("moles") * "180.16 g"/(1cancel("mole")) = color(green)("0.33 g") C_9H_8O_4
The percent composition of aspirin in your tablet will be
(0.33cancel("g"))/(0.50cancel("g")) * 100 = color(green)("66%")
SIDE NOTE Both answers ar rounded to two sig figs.
