All of these citations in the book of Hebrews rightly establish that Christ is the mediator of the New Covenant

 

Heb.1.2 (Psalm 2:8)
Heb. 1.3 (Psalm 110.1)
Heb 1.5 (Psalm 2:7)
Heb 1.6 (Psalm 97.7)
Heb 1.7 (Psalm 104.4 Lxx)
Heb 1.8-9 (Psalm 45.6-7)
Heb 1.10-12 (psalm 102.25-27 lxx)
Heb 1.13 (psalm 110.1)
Heb 1.14 (psalm 34.8, 91.11)
Heb 2.6-8 (psalm 8.4-6 lxx)
Heb 2.12 (psalm 22.22)
Heb. 3.7-11 (psalm 95.7-11)
Heb. 3.15 (psalm 95.7-8 lxx)
Heb 3.18 (psalm 95.11)
Heb 4.3.5 (psalm 95.11)
Heb 4.7 (psalm 95.7-8 lxx)
Heb 5.5 (psalm 2.7)
Heb.5.10 (psalm 110.4)
Heb 6.20 (psalm 110.4)
Heb. 7.3 (psalm 110.4)
Heb 10.5-7 (psalm 40.6-8)
Heb 10.8 (psalm 40.6)
Heb 10.9 (psalm 40.7)
Heb 10.13 (psalm 110.1)
Heb 10.30 (psalm 135.14)
Heb 11.3 (psalm 33.6,9)
Heb 11.13 (psalm 39.12)
Heb. 12.2 (psalm 110.1)
Heb 12.14 (psalm 34.14)
Heb 12.23 (psalm 50.6)
Heb 12.26 (psalm 68.8)
Heb 13.6 (118.6 lxx)
Heb 13.15 (psalm 50.14, 23)
Derived from the UBS Greek New Testament, as given in M. Bushell, Songs of Zion, 4th ed., p.53