The defendant, of no fixed address, dramatically changed his plea at Teesside Crown Court as he was due to go on trial this morning for the killing of Ashleigh Hall.
Chapman, 33, was told by Judge Peter Fox QC, the Recorder of Middlesbrough, he would serve a minimum of 35 years in prison before being considered for release.
The body of the 17-year-old childcare student was found dumped in a farmer's field near Sedgefield, County Durham, last October.
Miss Hall, from Darlington, was strangled and left near a known lovers' lane.
Chapman also pleaded guilty to failing to notify police of a change of address, as required by the sex offenders register.
Chapman, a convicted rapist, has a long history of sexual offending.
At 15 he was accused of sexual assault and four years later he was accused of raping a girl he had befriended. Both these allegations were discontinued.
But in December 1996, aged 19, he was jailed for seven years at Teesside Crown Court after being convicted of raping of two teenage prostitutes.
After his release, he was arrested by Cheshire Police in 2002 for the rape and kidnap of a prostitute in Ellesmere Port. Again the case was discontinued.
Today, at Teesside Crown Court, prosecutor Graham Reeds QC outlined how Chapman had lured Ashleigh to her death.
He had posed as a fictitious good-looking 19-year-old boy called 'Peter Cartwright' on Facebook - even posting fake pictures - to befriend the college student.
After becoming her Facebook friend and chatting on other social networking sites he arranged to meet her on the evening of October 25 last year.
Knowing that Ashleigh was expecting to meet a teenage boy and not a 33-year-old, Chapman led her to believe that Peter Cartwright's father would pick her up.
"The defendant used this handsome alter-ego to entice 17-year-old Ashleigh Hall into meeting him," Mr Reeds said.
"When she met him on October 25 last year, he kidnapped, raped and murdered her."
Mr Reeds said Chapman had spent the early part of October 25 visiting the area where he was to eventually take his victim.
"Having organised the 'Pete's dad' cover story, having researched the location of the intended crime scene and having just travelled to the area to check it, the defendant had now sprung the trap," the barrister told the court.
"All he had to do now was wait for her to fall into it."
The prosecutor went onto explain that Chapman drove Ashleigh to an area off the A177 at Thorpe Larches, near Sedgefield, County Durham.
"It was at this location the prosecution say the defendant raped Ashleigh Hall," Mr Reeds said.
"It is also possible that she was murdered at this location before her body was transported to the location at which it was discovered the next day.
"The evidence that was later recovered supports the proposition that when at Thorpe Larches lay-by Ashleigh Hall was forced to perform oral sex on the defendant.
"After removing her lower clothing she was gagged with duct tape wound around her face, bound by the forearms with tape and forced her into sexual intercourse.
"At some stage the bindings around her arms were removed - most likely to allow her to pull up and fasten her lower clothing.
"But then after the rape her arms were bound up again and further tape was put over her face, suffocating her to death."
Purely by chance - unaware Ashleigh was dead - police stopped Chapman's car the following day and he was arrested for several motoring offences.
Judge Fox passed a life sentence on Chapman and said he would serve at least 35 years imprisonment before being considered for parole.
"For it appears to me that you are, you were at the time, and have been for some considerable time, a very great danger to young women and, for what it is worth, I cannot foresee your release," he said.
"In my judgment, your killing was of such seriousness on its own, and in conjunction with other associated offences, that it falls clearly into the category of being particularly high."
Judge Fox said Chapman had carried out a "significant degree" of planning and pre-meditation in snaring Ashleigh.
"This was an evil scheme very carefully brought, and with considerable detail, to trap your victim," he told bearded Chapman.
"She was particularly vulnerable because of her age.
"Before you re-bound her so that she died from suffocation, all that led up must lead to mental and, in raping her, physical suffering."
