Query Post for 2 Category Pages

Hello There,

I have 2 menu that are category paged:
Events: http://cakedla.com/wordpress/?cat=5
Media: http://cakedla.com/wordpress/?cat=11

I’m trying to figure out the query post to set to view at 5 posts. I used the following code & added the navigation previous/next links at the bottom:
query_posts($query_string.'&category=5&showposts=11');

When I added the code above and clicked on next page navi, I get an error/404 page. Is there a php template file I need to add? Please help.

Thank you so much.