令氨水的體積為1ml，則：
質(zhì)量分數(shù)為25%的氨水的質(zhì)量為1ml×0.91g/cm³=0.91g，溶質(zhì)氨氣的質(zhì)量為0.91g×25%g．
質(zhì)量分數(shù)為5%的氨水的質(zhì)量為1ml×0.98g/cm³=0.98g，溶質(zhì)氨氣的質(zhì)量為0.98g×5%g．
所以混合后氨水的質(zhì)量分數(shù)為

0.91g×25%g+0.98g×5%g

0.91g+0.98g

×100%=14.63%＜15%．
故選C．