Answer
$0.0100\ M$
Work Step by Step
Since the number of moles is the same, before and after:
$(4.00\ mL\times 0.0250\ M)/10.0\ mL=0.0100\ M$
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.