Number + 秒钟
The measure word 秒钟 (miǎozhōng) is used to count seconds of time. It is the full form of the time unit, where 秒 alone also means "second," and 钟 reinforces the time-related meaning, parallel to 分钟 (minute) and 小时 (hour).
请等我三十秒钟,我马上就好。
qǐng děng wǒ sānshí miǎozhōng, wǒ mǎshàng jiù hǎo.
Please wait thirty seconds for me — I'll be ready right away.
运动员只用了九秒钟就跑完了百米。
yùndòngyuán zhǐ yòng le jiǔ miǎozhōng jiù pǎo wán le bǎi mǐ.
The athlete finished the hundred meters in just nine seconds.
倒计时还剩最后十秒钟。
dào jìshí hái shèng zuìhòu shí miǎozhōng.
There are only ten seconds left in the countdown.