web.config

I have two projects in solution. One is a website and other is a Class Library. Each has its own web.config.
I m trying to change the web.config present in the class library,  from a method in the default.asps.cs present in the website project.
System.Configuration.Configuration rootWebConfig = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("~/ClassLibrary1");
            System.Configuration.ConnectionStringSettings connString;
            if (0 < rootWebConfig.ConnectionStrings.ConnectionStrings.Count)
            {
                connString =
                    rootWebCon ...