It appears that OP's flight pushed at 09:54, 9 minutes late, and is due into the gate at IAD at 11:31, 6 minutes late. If the gate assignments of C19 for arrival and C5 hold and nothing intervenes, this is an easy transfer and OP ought to be at the PEK departure gate before boarding commences.
A general thought for the benefit of others in this situation is that I would not generally book such a short connection where other better connections are available (presuming that price is a constant). The key factor here is not whether 90 minutes is enough. It most certainly is at IAD. But, the consequences of a misconnect here were messy. At best, one is likely in lousier seats and at worst, delayed for at least 24 hours (all presuming availability) or rerouted to various other services.
So, always take a look at the consequences of a misconnect rather than simply whether the connection is doable.
Last edited by Often1; Jun 9, 2019 at 7:58 am