Answer
$C_{3}H_{2}O$
Work Step by Step
Number of moles of $CO_2$:
$257\ mg\div 44.01\ mg/mmol=5.84\ mmol$, $5.84\ mmol$ of C, $70.14\ mg$
Number of moles of water:
$35.0\ mg\div 18.015\ mg/mmol=1.94\ mmol$, $3.89\ mmol$ of H, $3.92\ mg$
Number of moles of oxygen:
$(105-70.14-3.92)\ mg\div 15.9994\ mg/mmol=1.93\ mmol$
O/C Ratio: $0.33=1/3$
H/C Ratio: $0.67= 2/3$
$C_{3}H_{2}O$
