EMB-175 First Officer reported encountering wake turbulence on descent into IAD in trail of a heavy jet. The encounter resulted in a Flight Attendant injury.

Narrative

We were descending via the HYPER8 arrival into IAD and unexpectedly encountered a moment of moderate turbulence - a rapid jolt over HYPER at about 10;000 ft. at 250 kts. Earlier during the descent; ATC notified us of a heavy aircraft also inbound to IAD and asked us if 7 miles separation was sufficient. We told the Controller we needed at least 10NM separation behind the heavy aircraft. We were then given radar vectors to provide more spacing between our aircraft and the heavy. We did not have the heavy aircraft in sight and could not determine its location on TCAS. Shortly after the incident; we picked up light icing but no turbulence before descending below the cloud layer. Immediately after the turbulence; one of the Flight Attendants called us informing us that the other Flight Attendant [was injured]. A Company mainline flight attendant took [the injured Flight Attendant's] jumpseat and [the injured Flight Attendant] took a passenger seat for the remainder of the flight. At the time of the incident; the Flight Attendants were standing in the aft galley. We immediately advised ATC; informed Dispatch; and continued the HYPER8 to land on Runway 19C. Paramedics met us at the gate.