British Empire:

In September 1982, Prime minister Margaret Thatcher travelled to Beijing to negotiate with the Chinese government on the future of Britain's last major and most populous overseas territory, Hong Kong. Under the terms of the 1842 Treaty of Nanking, Hong Kong Island itself had been ceded to Britain in perpetuity, but the vast majority of the colony was constituted by the New Territories, which had been acquired under a 99-year lease in 1898, due to expire in 1997. Thatcher, seeing parallels with the Falkland Islands, initially wished to hold Hong Kong and proposed British administration with Chinese sovereignty, though this was rejected by China. A deal was reached in 1984—under the terms of the Sino-British Joint Declaration, Hong Kong would become a special administrative region of the People's Republic of China, maintaining its way of life for at least 50 years. The handover ceremony in 1997 marked for many, including Charles, Prince of Wales, who was in attendance, "the end of Empire".

Please answer a question about this article. If the question is unanswerable, say "unanswerable". Which treaty had given Hong Kong to Britain?
Treaty of Nanking