When purchasing premixed concrete products in a bag and you want to only use a small amount it is advisable to dump the whole bag into a container and thoroughly  mix together the dry contents. Then remove the small amount you need.

Why,

When the contents are placed into a bag at the factory the premix is blended together at the factory. After it is bagged the bag then goes through a handling and shipping process until it reaches the store shelf then it is again handled until you use it. During all that process of handling the contents go through a process of vibration separating the contents by weight.

So when you just open the bag and just remove a small amount you may not get the proper mix ratio for the contents being used.

Found that out the hard way.