單位物質(zhì)的量的物質(zhì)所具有的質(zhì)量,稱(chēng)為摩爾質(zhì)量(molar mass),用符號(hào)M表示.當(dāng)物質(zhì)的質(zhì)量以克為單位時(shí),摩爾質(zhì)量的單位為g/mol,在數(shù)值上等于該物質(zhì)的相對(duì)原子質(zhì)量或相對(duì)分子質(zhì)量.對(duì)于某一純凈物來(lái)說(shuō),它的摩爾質(zhì)量是固定不變的,而物質(zhì)的質(zhì)量則隨著物質(zhì)的物質(zhì)的量不同而發(fā)生變化.