Chemistry X10 - problems and reactions is a free app, which is designed for Android.

This app contains over 8,000 chemical reactions that you can learn even without connecting to the internet. You can also add as many formulas as you need, and each of them contains all the necessary information.

Other things you can do with this app:

Arrange the coefficients by fitting or by method of electronic balance
Count molar masses of complex substances
Find reaction equations by entered part
Solve open chains
Study the periodic table