Answer
1.6 kg
Work Step by Step
The liquid oxygen is already at the boiling point. The additional heat causes it to evaporate. Assume the flask absorbs no heat.
$$Q=mL_v$$
$$m=\frac{Q}{L_v}=\frac{3.40\times10^5J}{2.1\times10^5J/kg}=1.6kg$$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.