乳酸的化学式量为:12×3+1×6+16×3=90;乳酸中碳、氢、氧三种元素的质量比:12×3:1×6:16×3=6:1:8;乳酸中碳元素的质量分数= 12×3 90 ×100%=40%.答案:90;6:1:8;40%.