Which structure acts as the pacemaker of the heart?

What does the given structural formula represent?

The molarity of a $0.2 \ N \ Na_2CO_3$ solution will be .......... $M$.

When a body of mass $1.0\, kg$ is suspended from a certain light spring hanging vertically,its length increases by $5\, cm$. By suspending a $2.0\, kg$ block to the spring,if the block is pulled through $10\, cm$ and released,the maximum velocity in $m/s$ is: (Acceleration due to gravity $g = 10\, m/s^2$)

The $pK_a$ values of four carboxylic acids are $4.76, 4.19, 0.23$,and $3.41$ respectively. The $pK_a$ value of the strongest carboxylic acid among them is

The velocity of a particle at time $t$ is given by the relation $v = 6t - \frac{t^2}{6}$. The distance traveled in $3$ seconds is,if $s = 0$ at $t = 0$.