将一定量的 Na2CO3和NaHCO3的混合物,跟足量盐酸反应共消耗0.8 mol HCl,生成0.5mol二氧化碳。求原混合物中NaHCO3的质量分数为多少?
34.6%
The reason ___ he didn’t attend the meeting yesterday was ___ his child had fallen ill.
A.that, why
B.why, that
C.why, why
D.that, that
质量为m的物体置于水平地面上,在物体顶端连一轻弹簧,弹簧处于自然长度.现用手将弹簧上端缓慢地竖直向上提起,使物体升高h,空气阻力不计,则此过程中( )
A.手对弹簧做的功为mgh
B.手对弹簧做的功小于mgh
C.手对弹簧做的功大于mgh
D.无法判定