Apache2's DocumentRoot and Tomcat5

Can anybody tell me if the following can be done ?
I have SLES9 running Apache2 with Tomcat5. The website that I need to
host is all made out of JSPs. There is no html !

Everything works great:
http://mydoamin.lan = Apache test page
http://mydomain.lan/webapp = Tomcat Servlet/JSP content

I need http://mydomain.lan to serve up the Tomcat Servlet/JSP content.

Right now if I replace DocumentRoot "/srv/www/htdocs" with
"/srv/www/tomcat/webapp" I get the following:

No Host matches server name 192.168.100.235
Your browser sent a request that this server could not understand.
Additionally, a 400 Bad Request error was encountered while trying to
use an ErrorDocument to handle the request.

Tomcat's mod_jk.co connector is working.

I just need to know where to start looking: Apache's httpd.conf,
default-server.conf... Tomcat server.xml...

Thanks.