Explain how sartre might defend existentialism against the claim that it leaves a moral vacuum

The claim that existentialism leaves a moral vacuum is a common one against this philosophy, and one that Sartre was aware of. Existentialism places freedom at the centre of human experience, and this includes the freedom from traditional moral views, which at first glance, could be worrisome for some people.

However, getting rid of moral traditions imposed by society is a choice: the choice to define your own morality. And by defining this morality, you replace old values with new ones. Therefore, there is no vacuum as there is no "empty space." When some social rules are removed from our ethical code, some other personal rules replace them. Sartre places great value on "authenticity." This is the creation of our own self in freedom. Through authenticity and freedom, we design ourselves, both as individuals and in society. Existentialism, therefore, does not imply the loss of ethics (and therefore, the lack of morals), but only the creation of a new ethics based on individual freedom and authenticity.

you can't answer this. more information is needed.