K
2K+2H2O=2KOH+H2
每2摩碱金属能生成1摩氢气,0.005克氢气为0.0025摩,故需耗碱金属0.005摩,0.195克/0.005摩=39克/摩,
K的原子量是39,所以该碱金属为K.