You can use
BEWARE the fancier "Wolfram|Alpha Pro" version since it costs money per month. I do not know the advantages over the $3 version, so I doubt they are worth it.
-----------Alternatively use the web browser on your mobile device and go to the website "http://m.wolframalpha.com/" to use the free "Wolfram|Alpha" web interface that has been re-organized for smaller phone screens instead of the regular "http://www.wolframalpha.com/" website which assumes you're using a large-screen computer.